Supercharging pressure control method for internal combustion engines

ABSTRACT

A method of controlling supercharging pressure in an internal combustion engine is disclosed. The supercharging pressure is controlled based on a basic control amount determined in dependence on operating conditions of the engine. When the transmission is in a lower speed position, the basic control amount is corrected so that the supercharging pressure is lower than a value assumed when the transmission is in a higher speed position. When the engine is in a feedback control mode, the basic control amount is corrected in response to the difference between the actual supercharging pressure and a desired value of same. The desired value of the supercharging pressure is set lower when the transmission is in the lower speed position than a value assumed when the transmission is in a higher speed position. When the transmission is in the lower speed position, the rising speed of the supercharging pressure in a transient states is set lower than a value assumed when the transmission is in a higher speed position. The basic control amount is corrected so as to decrease the supercharging pressure, and/or the desired supercharging pressure value is decreased, when the engine rotational speed changes from a state in which it increases to a state in which it decreases.

BACKGROUND OF THE INVENTION

This invention relates to a method of controlling supercharging pressure in an internal combustion engine equipped with a supercharger, and more particularly to an improved method of properly controlling the supercharging pressure when the transmission is in a predetermined position, or when the engine is in particular operating conditions.

A supercharging pressure control method for internal combustion engines has been proposed by the assignee of the present application, e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-129126, in which when the engine is in a transient operating condition in which the supercharging pressure abruptly increases, open loop control is executed, while when the engine is in a steady operating condition in which the supercharging pressure is relatively stable, feedback control is executed, whereby hunting in the supercharging pressure is prevented from taking place due to a time lag in the responsiveness of the control system to the actual change in the supercharging pressure which would take place if the feedback control were executed during the above transient operating condition of the engine, to thereby effect smooth control of the supercharging pressure.

However, the above proposed control method still has room for further improvement in respect of the accelerability of the engine and the controllability of the supercharging pressure as well as the durability or life of the engine when the transmission is in a predetermined gear position or in particular operating conditions.

First, torque acting upon component parts of the transmission of the engine varies depending on a gear position assumed by the transmission. The torque acting upon the component parts is very large when the transmission is in a first speed position Generally, the component parts should be designed so as to endure such large torque. However, since, as mentioned above, the torque is very large when the transmission is in the first speed position, if the supercharging pressure is controlled to a constant value irrespective of the gear position of the transmission, the transmission will have to be large in size.

Further, when the rotational speed of the engine temporarily drops at the time of shifting the gear position of the transmission, the supercharging pressure can overshoot due to a time lag in the operation of the supercharging pressure control system, in spite of the drop in the rotational speed of the engine, which makes it difficult to carry out stable control of the supercharging pressure.

Still further, in general, when the engine is accelerated with the transmission in a lower speed position (e.g. the first speed position), the rise rate of the engine rotational speed is larger than when the engine is accelerated with the transmission in a higher speed position. Accordingly, the rising speed of the supercharging pressure is higher in the former case. This is conspicuous especially at sudden standing-start of the vehicle. However, according to the aforesaid conventional method, the supercharging pressure is controlled in the transient condition always in the same manner, irrespective of the gear position of the transmission. Further, the control system has an inherent time lag in the responsiveness. Consequently, when the supercharging pressure is in the transient condition with the transmission in a lower speed position, the rising speed of the supercharging pressure exceeds the control speed of the system, so that the engine output is suddenly increased, resulting in spinning of the driving wheels of the vehicle and overboosting. Hence, good accelerability of the engine cannot be obtained.

Also, according to the conventional method, the desired supercharging pressure to which the supercharging pressure is to be controlled in feedback control mode during the steady condition is set at a single constant value, irrespective of the gear position of the transmission. As a result, the torque acting upon the transmission component parts will become large when the transmission is in the lower speed position during the steady condition of the supercharging pressure, which can cause overboosting and can badly affect the durability or life of the engine.

Further, it is desirable to stop supercharging the engine when the engine is in particular operating conditions, such as a condition in which the intake air temperature or the cooling water temperature is very low or very high, and a condition in which the supercharging pressure is very high. However, if the supercharging pressure is increased immediately when the engine has left such particular conditions, alternate supercharging and interruption thereof can be repeated at the boundary between such particular operating conditions and other operating conditions adjacent thereto, which renders the supercharging pressure unstable and can even badly affect the durability of the engine.

Furthermore, if the supercharging pressure is increased at the start of the engine in cold weather where the engine operation is unstable, it will cause abnormal combustion within the combustion chamber due to increased charging efficiency. Therefore, conventionally, the supercharging pressure is decreased at the start of the engine in cold weather and before the engine is warmed up. However, the predetermined temperature for determining whether or not the engine has been warmed up is set at a relatively low value corresponding to the temperature of the engine before being warmed up. Therefore, the supercharging pressure starts to be increased before completion of the warming-up of the engine when the engine temperature exceeds the predetermined temperature, and thereafter it is further increased with an increase in the engine rotational speed. As a result, the engine can be brought into a high load condition before being warmed up, also adversely affecting the durability of the engine.

Also, in an internal combustion engine with a supercharger in general, an intercooler is arranged in the intake pipe downstream of the supercharger. The cooling effect of the intercooler varies depending upon running conditions of the vehicle. For example, when the ambient air temperature is low, or when the vehicle is running at a high speed, the cooling effect of the intercooler increases so that the temperature of intake air supplied to the engine becomes too low, which results in an excessive increase in the charging efficiency of the intake air and hence overload on the engine. This is also undesirable to the durability of the engine,

To eliminate this disadvantage, it has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 60-128930, to decrease the supercharging pressure by a predetermined amount when the intake air temperature is extremely low.

However, in actuality, even when the intake air temperature is low, the engine is not overloaded if its rotational speed is low. On the contrary, if the supercharging pressure is decreased irrespective of the engine rotational speed, merely on condition that the intake air temperature is low, the supercharging pressure will slowly rise at the start of the engine, resulting in insufficient supercharging effect.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a supercharging pressure control method for internal combustion engines, which is capable of controlling the supercharging pressure so as to decrease same without causing an abrupt increase therein or overboosting when the transmission is in a predetermined lower speed position or in a state where the gear position thereof is being shifted, thereby enhancing the accelerability of the engine, the controllability of the supercharging pressure, and the durability or life of the engine.

It is a further object of the invention to provide a supercharging pressure control method, which is capable of controlling the supercharging pressure in a stable manner upon transition of the engine operating condition from a particular operating condition to a non-particular operating condition, thereby enhancing the durability or life of the engine.

Another object of the invention is to enhance the durability or life of the engine by properly controlling the supercharging pressure during warming-up of the engine. A still further object of the invention is to enhance the supercharging effect at the start of the engine without degrading the durability of the engine.

According to the invention, there is provided a method of controlling supercharging pressure in an internal combustion engine having a transmission and a supercharger, wherein the supercharging pressure created by the supercharger is controlled based on a basic control amount determined in dependence on operating conditions of the engine, the basic control amount being corrected in response to a difference between an actual value of the supercharging pressure and a desired value of same, when the engine is in an operating condition in which the supercharging pressure is controlled in feedback control mode.

According to a first aspect of the invention, the method is characterized by comprising the following steps:

(1) detecting a gear position of the transmission; and

(2) when the detected gear position is a predetermined lower speed position, correcting the basic control amount so that the supercharging pressure is lower than a value assumed when the detected gear position is a higher speed position.

According to a second aspect of the invention, the method is characterized by comprising the following steps:

(1) detecting a gear position of the transmission; and

(2) when the detected gear position is a predetermined lower speed position, correcting the desired value of the supercharging pressure to a value is set lower than a value assumed when the detected gear position is a higher speed position.

The predetermined gear position of the transmission of the first and second aspects may be a first speed position.

The step (2) of the first and second aspects may be executed when the engine is in a predetermined operating condition, which is a state in which intake pressure in the engine is higher than a predetermined value, which may be determined in dependence on the rotational speed of the engine.

According to a third aspect of the invention, the method is characterized by comprising the following steps:

(1) detecting a rising speed of the supercharging pressure in a transient state;

(2) detecting a gear position of the transmission; and

(3) when the detected gear position is a predetermined lower speed position, correcting the rising speed of the supercharging pressure in the transient state to a value lower than a value assumed when the detected gear position is a higher speed position.

The predetermined gear position of the transmission according to the third aspect may be a first speed position.

According to a fourth aspect of the invention, the method is characterized by comprising the following steps:

(1) detecting a change in the rotational speed of the engine; and

(2) correcting the basic control amount so as to decrease the supercharging pressure, when the rotational speed of the engine changes from a state in which it increases to a state in which it decreases.

According to a fifth aspect of the invention, the method is characterized by comprising the following steps:

(1) detecting a change in the rotational speed of the engine; and

(2) decreasing the desired value of the supercharging pressure when the rotational speed of the engine changes from a state in which it increases to a state in which it decreases.

The step (2) of the fifth aspect may be executed when the opening of a throttle valve of the engine, the rotational speed of the engine, and intake pressure in the engine exceed respective predetermined values.

According to a sixth aspect of the invention, the method is characterized by comprising the following steps:

(1) determining whether or not the engine is in a particular operating condition;

(2) when the engine is in the particular operating condition, setting the control amount so that the supercharging pressure is lower than a value assumed when the engine is in an operating condition other than the particular operating condition; and

(3) when the engine has left the particular operating condition, maintaining the control amount set in the step (2) so that the supercharging pressure is maintained at a lowered value over a predetermined time period after the engine has left the particular operating condition.

The particular operating condition of the engine may be a condition in which the engine is in a cold state, such as a condition in which the temperature of engine cooling water temperature is below a predetermined value, and a condition in which the temperature of intake air in the engine is below a predetermined value.

The method of the sixth aspect may include the step of determining whether or not the rotational speed of the engine is above a predetermined value, wherein the step (3) is executed when the rotational speed of the engine is above the predetermined value.

The predetermined period of time may correspond to a period of time required for warming up the engine.

In the sixth aspect, the control amount may be determined by a basic control amount and a correction value, the method including the steps of:

setting the correction value to an initial value dependent on the rotational speed of the engine when the predetermined period of time elapses, and holding the correction value at the initial value over a second predetermined period of time, to thereby correct the basic control amount; and

gradually returning the control amount to a value assumed when the engine is in an operating condition other than the particular operating condition, after the second predetermined period of time elapses.

According to a seventh aspect of the invention, the method is characterized by comprising the following steps:

(1) detecting the temperature of intake air in an intake pipe of the engine downstream of intake air-cooling means arranged in the intake pipe downstream of the supercharger of the engine;

(2) detecting the rotational speed of the engine; and

(3) when the detected temperature of intake air is below a predetermined value, and at the same time the detected rotational speed of the engine is above a predetermined value, setting the supercharging pressure lower than a value assumed when the rotational speed of the engine is below the predetermined value.

The above and other objects, features, and advantages of the invention will be more apparent from the ensuing detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the whole arrangement of the intake system and the exhaust system of an internal combustion engine to which is applied the method according to the invention;

FIG. 2 is an enlarged longitudinal cross-sectional view of a variable capacity turbocharger in FIG. 1;

FIG. 3 is a transverse cross-sectional view taken on line III--III of FIG. 2;

FIG. 4 is a transverse cross-sectional view taken on line IV--IV of FIG. 2;

FIGS. 5AI to 5AIII (collectively referred to as FIG. 5(A) and 5BI to 5BII (collectively referred to as FIG. 5(B) constitute a flowchart showing a main routine for controlling an electromagnetic control valve in FIG. 1, according to a first embodiment of the invention;

FIG. 6 is a flowchart showing a subroutine for selecting a time period to be counted by a timer;

FIG. 7 is a graph showing the relationship between a high supercharging pressure-discriminating value P_(2HG) and the engine rotational speed Ne;

FIG. 8 is a flowchart showing a subroutine for subtraction from a basic duty ratio and from desired supercharging pressure, which is executed when the transmission is in the first speed position;

FIG. 9 is a diagram showing a predetermined operating zone to be discriminated in the subroutine shown in FIG. 8;

FIG. 10 is a flowchart showing a subroutine for subtraction from the basic duty ratio and from the desired supercharging pressure, which is executed when the transmission is in a position other than the first speed position;

FIG. 11 is a flowchart showing a subroutine for determining a correction coefficient K_(DOWN) applied at the time of departure from a particular operating condition of the engine;

FIG. 12 is a flowchart showing a subroutine for determining a decremental value D_(T) ;

FIGS. 13 (a) 13(b) and 13(c), collectively referred to as FIG. 13, constitute a diagram showing a map of the decremental value;

FIG. 14 is a flowchart showing a subroutine for determining an incremental value D_(TRB) ;

FIGS. 15(a), 15(b) and 15(c), collectively referred to as FIG. 15, are diagrams showing maps of D_(TRB) ;

FIGS. 16(a), 16(b) and 16(c), collectively referred to as FIG. 16, are similar diagrams to FIGS. 15(a), 15(b) and 15(c) , showing maps of a decremental value ΔP_(2ST) ;

FIGS. 17(a), 17(b) and 17(c), collectively referred to as FIG. 17, are similar diagrams to FIGS. 15(a), 15(b) and 15(c), showing maps of a decremental valve ΔP_(2FB) ;

FIG. 18 is a diagram showing a map of a duty ratio D_(SCRB) to be determined depending on the engine rotational speed N_(E) ;

FIG. 19 is a flowchart showing a subroutine for determining feedback coefficients for determining, respectively, a proportional control term and an integral control term;

FIG. 20 is a diagram showing a change in the intake pressure, which can take place when the gear position of the transmission is shifted;

FIG. 21 is a diagram showing changes in a duty ratio and supercharging pressure, which can take place when the control mode is shifted from the open loop control mode to the feedback control mode;

FIGS. 22A and 22B, collectively referred to as FIG. 22, constitute a flowchart showing a main routine for controlling an electromagnetic valve in FIG. 1;

FIGS. 23A, 23B and 23C, collectively referred to as FIG. 23, constitute shows a variation of the first embodiment of the invention, showing a flowchart of a main routine for controlling the electromagnetic control valve;

FIGS. 24A and 24B, collectively referred to as FIG. 24, constitute a flowchart showing a main routine for controlling the electromagnetic control valve according to a second embodiment of the invention;

FIG. 25 is a diagram showing a map of a basic duty ratio D_(M) ;

FIG. 26 is a flowchart showing a subroutine for determining the gear position of the transmission;

FIG. 27 is a diagram showing a table of a predetermined value V_(F) of the vehicle speed, applied to the subroutine of FIG. 26;

FIG. 28 is a diagram showing a map of an intake air temperature-dependent correction coefficient K_(TATC) ;

FIGS. 29A and 29B, collectively referred to as FIG. 29, constitute a flowchart showing a subroutine for determining an open loop control region, which is executed at a step S106 in FIG. 24;

FIG. 30 is a diagram showing a table of a first decremental value ΔP_(BSD) to be applied when the transmission is in a position other than the first speed position;

FIG. 31 is a diagram showing a table of a second decremental value ΔP_(BFB) to be applied when the transmission is in a position other than the first speed position;

FIG. 32 is a diagram showing a table of a subtraction term D_(T) to be applied when the transmission is in a position other than the first speed position;

FIG. 33 is a diagram showing a table of a subtraction term D_(FT) to be applied when the transmission is in the first speed position;

FIG. 34 is a diagram showing a map of a desired value P_(BREF) of supercharging pressure;

FIG. 35 is a diagram showing a table of a constant K_(P) for a proportional control term K_(P) ;

FIG. 36 is a diagram showing a table of a constant K_(I) for an integral control term K_(I) ;

FIG. 37 is a diagram showing a map of a learned correction coefficient K_(MOD) ;

FIG. 38 is a diagram showing the relationship between the intake pressure P_(B) and the supercharging pressure control; and

FIG. 39 is a graph showing a supercharging pressure characteristic depending on the gear position of the transmission, obtained by the second embodiment of the invention.

DETAILED DESCRIPTION

The invention will now be described in detail with reference to the drawings showing embodiments thereof.

Referring first to FIGS. 1 through 4, there is illustrated a supercharging pressure control system for an internal combustion engine, to which is applied the method according to the invention. The engine is a multiple-cylinder type which has a cylinder block E with a plurality of cylinders each provided with an intake port, neither of which is shown. Connected to the intake port of each cylinder is an intake manifold 1, to which are connected an intake pipe 2, a throttle body 3, an intercooler 4, a variable capacity type turbocharger 5, and an air cleaner 6 in the order mentioned. Each cylinder has an exhaust port, not shown, to which is connected an exhaust manifold 7. Connected to the exhaust manifold 7 is an exhaust pipe 8 with the turbocharger 5 arranged across an intermediate portion thereof. A three-way catalytic converter 9 is arranged across the exhaust port at a location downstream of the turbocharger 5. Fuel injection valves 10 are mounted in the intake manifold 1 at locations close to the intake ports of the respective cylinders for injecting fuel toward the intake ports.

The turbocharger 5 is provided with a water jacket 11, an inlet of which is connected in parallel with an outlet of a water pump 13, together with an inlet of the intercooler 4. The water jacket 11 and the intercooler 4 have their outlets connected to the radiator 12. The radiator 12 is provided in addition to a radiator, not shown, for cooling coolant supplied into the interior of the cylinder block E of the engine.

The structure of the variable capacity type turbocharger 5 will now be explained with reference to FIGS. 2-4. The turbocharger 5 comprises a compressor casing 14, a back plate 15 closing a rear side of the compressor casing 14, a main shaft 16, a bearing casing 17 supporting the main shaft 16, and a turbine casing 18.

A scroll passage 19 is defined between the compressor casing 14 and the back plate 15, and an axially extending inlet passage 20 is defined through a central portion of the compressor casing 14. A compressor wheel 21 is mounted on an end of the main shaft 16 at a central portion of the scroll passage 19 and at an inner end of the inlet passage 20.

The compressor casing 14 and the back plate 15 are fastened together by means of bolts 22. The bearing casing 17 is joined to the back plate 15 at a central portion thereof. The bearing casing 17 is formed therein with a pair of bearing holes 23, 24 in coaxial and spaced relation to each other, through which the main shaft 16 extends. Interposed between the main shaft 16 and the bearing holes 23, 24 are radial bearings 25, 26 rotatably supporting the main shaft 16 against the bearing casing 17. Interposed between a stepped shoulder 16a of the main shaft 16 facing toward the compressor wheel 21 and the compressor wheel 21 are a collar 27, a thrust bearing 28, and a bushing 29 in the order mentioned as viewed from the stepped shoulder 16a side. By fastening a nut 30 threadedly fitted on an end portion of the main shaft 16 against an outer end of the compressor wheel 21, the main shaft 16 is located in its proper axial position and at the same time the compressor wheel 21 is mounted onto the main shaft 16.

A lubricating oil inlet port 32 is formed in a lateral side wall of the bearing casing 17 and connected to a lubricating oil pump, not shown, and a lubricating oil passage 33 is formed in the bearing casing 17 for guiding lubricating oil from the lubricating oil inlet port 32 to the radial bearings 25, 26 as well as to the thrust bearing 28. The bearing casing 17 has the other lateral side wall formed with a lubricating oil drain port 34 for draining lubricating oil. The drained oil is collected into an oil sump, not shown.

The bushing 29 extends through a through hole 35 formed in a central portion of the back plate 15. A seal ring 36 is interposed between the bushing 29 and the through hole 35 to prevent lubricating oil from flowing from the thrust bearing 28 to the compressor heel 21. A guide plate 37 is interposed between the back plate 15 and the thrust bearing 28, through which the bushing 29 extends, so that lubricating oil flowing from the thrust bearing 28 is guided by the guide plate 37 while it is splashed in the radially outward direction. A free end portion of the guide plate 37 is curved so as to smoothly guide the lubricating oil into the lubricating oil drain port 34.

The bearing casing 17 is further formed therein with the aforementioned water jacket 11 disposed around the main shaft 16, a water supply port 38 for guiding water or coolant from the water pump 13 shown in FIG. 1 to the water jacket 11, and a water drain port 39 for guiding water from the water jacket 11 to the radiator 12 shown in FIG. 1. The water jacket 11 has a portion closer to the turbine casing 18 which is shaped in the form of an annulus surrounding the main shaft 16, and a portion above the lubricating oil drain port 34 and the main shaft 16, which has a generally U-shaped section in a manner downwardly diverging along the main shaft 16 as shown in FIG. 4. The water supply port 38 communicates with a lower portion of the water jacket 11, while the water drain port 39 communicates with an upper portion of the water jacket 11.

The turbine casing 18 is formed therein with a scroll passage 41, an inlet passage 42 tangentially extending from the scroll passage 41, and an outlet passage 43 axially extending from the scroll passage 41.

The bearing casing 17 and the turbine casing 18 are joined together with a back plate 44 held therebetween. That is, the two members are fastened together by tightening nuts 47 via rings 46 onto respective stud bolts 45 screwed in the turbine casing 18, with a radial flange 44a at the periphery of the back plate clamped between the two members.

Secured to the back plate 44 is a stationary vane member 48 which divides the interior of the scroll passage 41 into a radially outer passage 41a, and a radially inner or inlet passage 41b. The stationary vane member 48 comprises a cylindrical hub portion 48a coaxially fitted in the outlet passage 43 via a seal ring 51, an annular radial portion 48b radially outwardly extending from an axially intermediate portion of the cylindrical hub portion 48a, a plurality of, e.g. four stationary vanes 49 axially extending from an outer peripheral edge of the annular radial portion 48b and secured to the back plate 44 by means of bolts 52. A turbine wheel 50 is accommodated within the stationary vane member 48, which is secured on the other end of the main shaft 16.

The stationary vanes 49 are circumferentially arranged at equal intervals, each being arcuate in shape. Disposed between adjacent stationary vanes 49 are movable vanes 54 with one ends thereof secured to respective rotary shafts 53 rotatably supported by the back plate 44 with their axes extending parallel with that of the main shaft 16. The movable vanes 54 act to adjust the opening area of spaces (hereinafter called "the space area") between adjacent stationary and movable vanes 49, 54.

Each movable vane 54 is also arcuate in shape, with almost the same curvature as the stationary vanes 49, and pivotable between a fully closed position shown by the solid line in FIG. 3 and a fully open position shown by the broken line in the figure. The rotary shafts 53 are operatively connected to an actuator 60 in FIG. 1 by means of a link mechanism 55 disposed between the back plate 44 and the bearing casing 17 so that the movable vanes 54 are simultaneously controlled to open and close by the actuator 60.

Interposed between the back plate 44 and the bearing casing 17 is a shield plate 56 extending along a rear end face of the turbine wheel 50, for preventing the heat of exhaust gases from the engine flowing in the inlet passage 41b from being directly transmitted to the interior of the bearing casing 17. A plurality of annular grooves 58 are formed as labyrinth grooves in the outer peripheral surface of the main shaft 6 at a location corresponding to a through hole 57 formed in the bearing casing 17 and penetrated by an end of the main shaft 16. These grooves 58 serve to prevent exhaust gases from leaking into the bearing casing 17.

With the above described arrangement, exhaust gas emitted from the engine cylinder block E flows into the radially outer passage 41a through the inlet passage 42, and then flows into the inlet passage 41b at a flow rate corresponding to the space area between the movable vanes 54 and the stationary vanes 49, which is determined by the angle of the movable vanes 54. As the exhaust gas flows into the inlet passage 41b, it drives the turbine wheel 50 to rotate. Then, the gas is discharged through the outlet passage 43. As the space area between the movable and stationary vanes 54, 49 decreases, the rotational speed of the turbine wheel 50 and hence that of the main shaft 16 becomes higher, whereas as the opening area increases, the rotational speed becomes lower. The rotation of the turbine wheel 50 causes rotation of the compressor wheel 21 so that air introduced into the inlet passage 20 through the air cleaner 6 is compressed by the rotating compressor wheel 21 to be forced to pass through the scroll passage 19 toward the intercooler 4. When the movable vanes 54 are moved into the radially outermost position so that the space area between the movable and stationary vanes 54, 49 becomes the minimum, the supercharging pressure becomes the maximum, whereas when the movable vanes 54 assumes the radially innermost position and hence the opening area becomes the maximum, the supercharging pressure becomes the minimum.

Water supplied into the water jacket 11 serves to prevent the temperature of the bearing casing 17 from becoming excessively high due to increased temperature of air compressed by the turbocharger 5, while water supplied to the intercooler 4 serves to prevent increase of the intake air temperature.

Referring again to FIG. 1, the actuator 60, which drives the movable vanes 54 of the turbocharger 5, comprises a housing 61, a diaphragm dividing the interior of the housing 61 into a first pressure chamber 62 and a second pressure chamber 63, a return spring 65 interposed between the housing and the diaphragm 64 and urging the diaphragm 64 in a direction causing the first pressure 62 to contract, and a driving rod 66 airtightly and movably extending through the housing 61, with one end thereof connected to the diaphragm 64 and the other end to the link mechanism 55. The driving rod 66 and the link mechanism 55 are connected to each other in such a manner that when the driving rod 66 is moved by the diaphragm 64 which is displaced in a direction causing the second pressure chamber 63 to contract, the movable vanes 54 are radially inwardly pivoted in the turbine casing 18 to increase the space opening area between the movable and stationary vanes 54, 49.

The first pressure chamber 62 is connected to a portion of the intake passage between the turbocharger 5 and the intercooler 4 via a regulator 67, a restriction 68, and an electromagnetic control valve 69, to be supplied with supercharging pressure P₂ therefrom, and is also connected to another portion of the intake passage between the air cleaner 6 and the turbocharger 5. The electromagnetic control valve 69 is a normally-closed duty control type with a solenoid 70. As the valve-closing duty ratio for the solenoid 70 becomes smaller, the pressure within the first pressure chamber 62 increases, which is transmitted through the driving rod 66 and the link mechanism 55 to cause the movable vanes 54 to be radially inwardly pivoted, i.e. toward the closing side. The second pressure chamber 63 is connected to a portion of the intake passage downstream of the throttle body 3 through a check valve 71 and an electromagnetic valve 72 to be supplied with intake pressure P_(B) therefrom. The electromagnetic valve 72 is a normally-closed type which becomes open when its solenoid 73 is energized. When the valve 72 is open, intake pressure P_(B) is supplied into the second pressure chamber 63 so that the actuator 60 drives the movable vanes 54 to be radially inwardly displaced.

The electromagnetic valves 69, 72 are controlled by an electronic control unit (control means) C, to which are connected a water temperature sensor S_(W) for sensing the temperature T_(W) of cooling water in a water jacket, not shown, provided in the engine cylinder block E, an intake air temperature sensor S_(A) for sensing the temperature T_(A) of intake air in the intake passage downstream of the intercooler 4, an intake pressure sensor S_(PA) for sensing intake pressure P_(A) in the intake passage at a location between the air cleaner 6 and the turbocharger 5, a supercharging pressure sensor S_(P2) for sensing supercharging pressure P₂ in the intake passage at a location between the turbocharger 5 and the intercooler 4, an intake pressure sensor S_(PB) for sensing intake pressure P_(B) in the intake passage downstream of the throttle body 3, an engine speed sensor S_(N) for sensing the rotational speed N_(E) of the engine, a throttle valve opening sensor S_(TH) for sensing the valve opening θ_(TH) of a throttle valve 74 within the throttle body 3, a vehicle speed sensor S_(V) for sensing the speed V of a vehicle in which the engine is installed, and a gear position sensor S_(S) for sensing the gear position of an automatic transmission connected to the engine. The control unit C operates in response to the input signals from these sensors to control the energization and deenergization of the solenoids 70, 73 of the electromagnetic valves 69, 72.

Next, the manner of control by the control unit C will be described below. First, the control of duty ratio of the solenoid 70 of the electromagnetic control valve 69 will be described with reference to a main routine shown in FIGS. 5A and 5B, according to a first embodiment of the invention. The valve-closing duty ratio D_(OUT) represents the ratio of valve-closing time to the time period of one cycle over which the valve 69 is opened and closed. Therefore, as the duty ratio D_(OUT) is larger, the opening degree of the movable vanes 54 is decreased, and D_(OUT) =0% corresponds to the maximum opening degree of the movable vanes 54 while D_(OUT) =100% corresponds to the minimum opening degree of same.

At a step S1, it is determined whether or not the engine is in starting mode, i.e. the engine is cranking. If the engine is in starting mode, the program proceeds to a step S2, where a timer t_(BTWC) is reset. The timer t_(BTWC) is for counting a time period t_(BTWCO) (e.g. 98 sec.) required to determine that warming-up of the engine has been completed. Then, at a step S3, a t_(FBDLY) timer for counting a time period t_(FBDLY) by which the start of the feedback control is delayed is reset. And then, the duty ratio D_(OUT) is set to 0%, i.e. the electromagnetic control valve 69 is fully opened to set the maximum space area between the movable vanes 54 and the stationary vanes 49 (step S4). The engine is unstable during cranking, and if supercharging pressure is introduced into combustion chambers while the engine is in such an unstable state, the engine will be more unstable. Therefore, in the above step S2, the space area between the movable vanes 54 and the stationary vanes 49 is made the maximum to thereby prevent supercharging pressure from being introduced into the combustion chambers. Further, a driver of the vehicle does not demand supercharging of intake air during cranking, and therefore it is not necessary to reduce the space area between the movable vanes 54 and the stationary vanes 49. At a step S5, the duty ratio D_(OUT) is outputted.

The time period t_(FBDLY) is calculated in a manner shown in FIG. 6. Depending on the change rate ΔP₂ in supercharging pressure P₂, one of three time periods t_(FBDLY1), t_(FBDLY2), and t_(FBDLY3) is selected as t_(FBDLY). The change rate ΔP₂ is calculated as the difference (ΔP₂ =P_(2n) -P_(2n) -6) between the supercharging pressure P_(2n) detected in the present loop and the supercharging pressure P_(2n-6) detected in the sixth loop before the present loop. More specifically, the main routine shown in FIGS. 5A and 5B is carried out in synchronism with generation of TDC signal pulses. However, since the change rate ΔP₂ in supercharging pressure P₂ between two adjacent TDC signal pulses is too small for accurate detection of the change rate ΔP₂, the difference between the P_(2n) detected in the present loop and the P_(2n-6) detected in the sixth loop before the present loop is calculated in order to detect the supercharging characteristic or the change rate ΔP₂ more accurately. A predetermined lower change rate ΔP_(2PTL) and a predetermined higher change rate ΔP_(2PTH) are provided which are determined in accordance with the engine rotational speed N_(E). If ΔP₂ ≦ΔP_(2PTL), t_(FBDLY1) is selected, if ΔP_(2PTL) <ΔP₂ ≦ΔP_(2PTH), t_(FBDLY2) is selected, and if ΔP_(2PTH) <ΔP₂, t_(FBDLY3) is selected. Further, the three time periods are in the relationship of t_(FBDLY1) <t_(FBDLY2) <t_(FBDLY3). Therefore, when the change rate ΔP₂ is small, i.e. the supercharging pressure undergoes a gentle change, the delaying time is set to a smaller value, and when the change rate ΔP₂ is great, i.e. the supercharging pressure undergoes a drastic change, the delaying time is set to a larger value. This makes it possible to set the delaying time period t_(FBDLY) to an appropriate value when the operating mode is shifting from open loop mode to feedback control mode, to thereby positively prevent occurrence of hunting of the supercharging pressure during the transitional state of the operating mode.

If it is determined at the step S1 that the engine is not in the starting mode, the program proceeds to a step S6, where it is determined whether or not the TDC signal pulse inputted in the present loop is the first one after control in a basic mode has been started, that is, the present loop is the first loop after the basic mode control has been started. If it is determined that the present loop is the first loop, the program proceeds to a step S7, whereas the present loop is not the first loop, it proceeds to a step S11. At the step S7, it is determined whether or not the intake air temperature T_(A) is above a predetermined lower value T_(AL) (e.g. -8° C.). If T_(A) >T_(AL), the program proceeds to a step S8, whereas if T_(A) 23 T_(AL), it proceeds to a step S10. At the step S8, it is determined whether or not the cooling water temperature T_(W) is above a predetermined lower value, (e.g. 60° C.). If T_(W) >T.sub. WL, the program proceeds to a step S9, whereas if T_(W) ≦T_(WL), it proceeds to the step S10.

At the step S9, the timer t_(BTWC) is set to a value FF larger than the predetermined time period t_(BTWCO) (e.g. 96 sec.), followed by proceeding to a step S13, while at the step S10, the timer t_(BTWC) is reset, followed by the program proceeding to the step S3.

That is, if T_(A) >T_(AL) and at the same time T_(W) >T_(WL), it is determined that the engine is in an operating condition after completion of warming-up thereof, so that the timer t_(BTWC) is set to the time period FF larger than the predetermined time period t_(BTWCO), whereas if at least one of the conditions of T_(A) ≦T_(AL) and T_(W) ≦T_(WL) is fulfilled, the timer t_(BTWC) is reset to start counting. Thus, the time period for determining that the warming-up of the engine has been completed starts to be counted after the basic mode control has been started.

At the step S11, it is determined whether or not the intake air temperature T_(A) is below the predetermined lower value T_(AL). If T_(A) <T_(AL), the program proceeds to a step S2, while if T_(A) ≧T_(AL), the program proceeds to a step S12. At the step S12, it is determined whether or not the cooling water temperature T_(W) is below the predetermined lower value T_(WL). If T_(W) <T_(WL), the program proceeds to the step S2, while if T_(W) ≧T_(WL), the program proceeds to the step S13. That is, if it is determined at the step S6 that the present loop is not the first loop, the intake air temperature T_(A) and the cooling water temperature T_(W) are compared with the respective predetermined values at the steps S11 and S12, followed by the program proceeding to the step S2 or S13 in accordance with the results of respective determinations.

The possible operating conditions of the engine which satisfy T_(W) <T_(WL) and T_(A) <T_(AL) are, for example, those in which the engine is at an early stage of starting or the ambient air temperature is very low.

At the early stage of starting, the operation of the engine is unstable, while when the ambient air temperature is very low, the intake air density is high to increase the charging efficiency, which may result in abnormal combustion of the engine. If supercharging pressure is introduced into the combustion chambers under such a cold state of the engine, the operation of the engine may be even more unstable, and the abnormal combustion may be promoted. Further, at an extremely low temperature, there is a possibility of malfunctioning of the electromagnetic valve 69, that is, the electromagnetic valve 69 may not behave in accordance with instructions from the control unit C. Therefore, if T_(W) <T_(WL) and/or T_(A) T_(AL), the program proceeds through the steps S2, S3 to the step S4 to set D_(OUT) to 0%.

At the step S13, it is determined whether or not the engine rotational speed N_(E) is above a predetermined value N_(DO) (e.g. 5000 rpm). If N_(E) >N_(DO), the program proceeds to a step S14, while if N_(E) ≦N_(DO), the program skips over the step S14 to a step S15. At the step S14, it is determined whether or not the timer t_(BTWC) has counted up the predetermined time period t_(BTWCO) required to determine that warming-up of the engine has been completed. If t_(BTWC) >t_(BTWCO), the program proceeds to the step S15, while if t_(BTWC) ≦t_(BTWCO), the program proceeds to the step S3.

As described above, if the cooling water temperature T_(W) is below the predetermined lower value T_(WL), the duty ratio D_(OUT) is set to 0% to thereby decrease the supercharging pressure P₂, while even if the cooling water temperature T_(W) is above the predetermined lower value T_(WL), when the engine rotational speed N_(E) is above the predetermined value N_(DO), D_(OUT) is maintained at 0% until the predetermined time period t_(BTWCO) elapses. Consequently, even if the engine rotational speed is increased during warming-up of the engine, the supercharging pressure is not increased.

At the step S15, it is determined whether or not the intake air temperature T_(A) is above a predetermined higher value T_(AH) (e.g. 100° C.). If T_(A) >T_(AH), the program proceeds to the step S3, while if T_(A) ≦T_(AH), the program proceeds to a step S16.

At the next step S16, it is determined whether or not the engine coolant temperature T_(W) exceeds a predetermined higher value T_(WH) (e.g. 120° C.). If T_(W) >T_(WH), the program proceeds to the step S3. The possible operating conditions which satisfy T_(A) >T_(AH) and T_(W) >T_(WH) are, for example, those in which the engine has been continuously operating under a high load condition, or the ambient air temperature is very high, or the engine coolant system of the engine cylinder block E is malfunctioning. Under such high temperature conditions of the engine, the intake air density is low to decrease the charging efficiency, which may also result in abnormal combustion such as misfiring. If supercharging pressure is introduced into the combustion chambers when the engine is under such unstable operating conditions, the engine operation will be made even more unstable. Therefore, at the step S4, the duty ratio D_(OUT) is set to 0. Further, when the ambient air temperature is very high, the inductance of the solenoid 70 is liable to change, so that it may behave differently from a predetermined behavior under normal induction conditions. Also for the purpose of avoiding this, the program proceeds to the step S4.

At the step S16, if T_(W) ≦T_(WH), the program proceeds to a step S17. At the step S17, it is determined whether or not supercharging pressure exceeds a predetermined high supercharging pressure-discriminating value P_(2HG) set as shown in FIG. 7. If P₂ >P_(2HG), the program proceeds to the step S3. If P₂ ≦P_(2HG), the program proceeds to a step S18. The predetermined high supercharging pressure-discriminating value P_(2HG) is set in accordance with the engine rotational speed N_(E). The value P_(2HG) is provided in order that the supercharging pressure may not be higher than a limit value of the amount of advancement of ignition timing above which knocking can take place, the limit value corresponding to the engine rotational speed N_(E) so as to ensure attainment of the maximum output of the engine immediately under the limit value. When the engine rotational speed N_(E) is in a low range, where the transmission is set into a low speed position, the torque which is applied to the transmission component parts increases, whereas when the engine rotational speed N_(E) is in a high engine rotational speed range, knocking can take place, adversely affecting the durability of the engine main body E. Therefore, P_(2HG) is set to values lower than a medium engine rotational speed range. If the supercharging pressure P₂ which exceeds the high supercharging pressure-discriminating value P_(2HG) is detected, the program proceeds through the step S3 to the step S4, where the duty ratio D_(OUT) is set to 0% whereby the supercharging pressure P₂ is decreased, and at the same time fuel injection is inhibited.

At the step S18, a basic duty ratio D_(M) is determined as a basic supercharging pressure control amount. The basic duty ratio D_(M) is searched from a map in accordance with the engine rotational speed N_(E) and the throttle valve opening θ_(TH), whereby it is made possible to accurately determine operating conditions of the engine. This is because it is impossible to accurately determine decelerating or transitional operating conditions of the engine by the use of the engine rotational speed N_(E) alone or the throttle valve opening θ_(TH) alone. In this embodiment, the throttle valve opening θ_(TH) is adopted as a parameter representative of load on the engine. However, it may be replaced by the intake pressure P_(B) or the fuel injection amount.

At a step S19, it is determined whether or not the automatic transmission is in a first speed position. If the automatic transmission is in the first speed position, the program proceeds to a step S20, and if the transmission is in a position other than the first speed position, the program proceeds to a step S21.

At the step S20, subtraction is effected from the basic duty ratio D_(M) in accordance with a subroutine shown in FIG. 8. More specifically, a predetermined operating zone is provided as shown by hatching in FIG. 9, which is determined by the engine rotational speed N_(E) and the intake pressure P_(B), in which zone subtraction from the basic duty ratio D_(M) should be effected. Depending on whether or not the operating condition of the engine is within this predetermined operating zone, it is determined whether or not subtraction should be effected from the basic duty ratio D_(M). In FIG. 9, the torque of the engine is determined based upon the engine rotational speed N_(E) and the intake pressure P_(B), and the border line of the predetermined operating zone indicates the maximum allowable torque amount applied to the gear shaft of the transmission when the transmission is in the first speed position. In other words, in order to prevent excessive load on the gear shaft when the transmission is in the first speed position, the torque of the engine in each operating region is monitored accurately by the use of the engine rotational speed N_(E) and the intake pressure P_(B). If the operating condition of the engine is outside the predetermined operating zone, the program proceeds to a step 22 without correcting the basic duty ratio D_(M) whereas if the operating condition of the engine is within the predetermined operating zone, it is determined whether or not a flag F is 0, i.e. the engine is in the feedback control mode. If the engine is in the open loop control mode, subtraction of D_(M) =D_(M) -D_(F) is carried out. If the engine is in the feedback control mode, subtraction of P_(2REF) =P_(2REF) -ΔP_(2REFF) is carried out. D_(F) is a predetermined decremental value, P_(2REF) is a desired value of supercharging pressure used in the feedback control mode, and ΔP_(2REFF) is also a predetermined decremental value.

At the step S2I, subtraction is effected from the basic duty ratio D_(M) in accordance with a subroutine shown in FIG. 10. More specifically, if the throttle valve opening θ_(TH) is above a predetermined value θ_(THOS), the engine rotational speed N_(E) is above a predetermined value N_(EOS), the intake pressure P_(B) is above a predetermined value P_(BOS), change rate ΔN_(E) of the engine rotational speed N_(E) detected in the last loop is positive, and the change rate ΔN_(E) of the engine rotational speed N_(E) detected in the present loop is negative, subtraction of D_(M) =D_(M) -D_(OS) is carried out in the open loop control mode, and subtraction of P_(2REF) =P_(2REF) -ΔP_(2REFOS) is carried out in the feedback control mode. Otherwise, the program proceeds to the step S22 without correcting the basic duty ratio D_(M). D_(OS) and ΔP_(2REFOS) are predetermined decremental values.

At the step S22 are searched a duty ratio correction coefficient K_(MODij), an atmospheric pressure-dependent correction coefficient K_(PATC) (0.8 to 1.0), and an intake air temperature-dependent correction coefficient K_(TATC) (0.8 to 1.3). The duty ratio correction coefficient K_(MODij) is searched from a map of the engine rotational speed N_(E) and the intake air temperature T_(A). As described later, the correction coefficient K_(MODij) is learned when the actual supercharging pressure P₂ is within a predetermined difference range about the desired supercharging pressure value, and renewed an up-to-date learned value. The initial value of the correction coefficient K_(MODij) is set to 1. The atmospheric pressure-dependent correction coefficient K_(PATC) is determined by the atmospheric pressure P_(A). The intake air temperature-dependent correction coefficient K_(TATC) is determined by the intake air temperature T_(A).

At a step S23 following the step 22, a correction coefficient K_(DOWN) is searched in accordance with a subroutine shown in FIG. 11. This subroutine is executed by interrupting the main routine shown in FIGS. 5A and 5B, in synchronism with generation of each pulse of the TDC signal. When the duty ratio D_(OUT) is 0%, a timer t_(DOWN) is reset. The correction coefficient K_(DOWN) is set to an initial value upon generation of the first TDC signal pulse after the duty ratio D_(OUT) becomes more than 0%. The initial value is determined depending on the engine rotational speed N_(E). When N_(E) exceeds a predetermined value N_(EA), e.g. 3000 rpm, the initial value is set to a value K_(DOWN1), e.g. 0.5, whereas when N_(E) is below the predetermined value N_(EA), it is set to a value K_(DOWN2), e.g. 0.6. Further, after the predetermined time period t_(DOWN), e.g. 5 sec., has elapsed, the correction coefficient K_(DOWN) is renewed by being increased by an additional value ΔK_(DOWN), e.g. 0.01 whenever each TDC signal pulse is generated. If the correction coefficient K_(DOWN) exceeds 1.0, it is set to 1.0.

The correction coefficient K_(DOWN) is determined as above is substituted into an equation for correcting the duty ratio D_(OUT) to control same so as to gently vary when the duty ratio is released from a condition in which it is forced to be 0% when the engine is in a particular operating condition in which the intake air temperature T_(A) and the cooling water temperature T_(W) are too high or too low, and the supercharging pressure P₂ is to high. More specifically, when the engine has returned to a normal operating condition from the particular condition in which D_(OUT) =0%, of D_(OUT) is immediately set to a normal value other than 0%, the D_(OUT) value will vary unstably as the engine shifts between the normal and particular operating conditions, resulting in unstable control of the supercharging pressure. In order to avoid such unstable control, the correction coefficient K_(DOWN) is gradually increased in each loop by e.g. 0.01, after the predetermined time period, e.g. five seconds have elapsed since the engine returned to the normal operating condition.

At the step S24, it is determined whether or not the throttle valve opening θ_(TH) is above a predetermined value θ_(THFB). This predetermined value θ_(THFB) is for determining whether the control mode should be shifted from the open loop control mode to the feedback control mode. By adopting the throttle valve opening θ_(TH) as the determining parameter, it is possible to accurately determine whether the driver of the vehicle demands acceleration, i.e. supercharging. If θ_(TH) ≦θ_(THFB), i.e. if the open loop control is to be continued, the t_(BDLY) timer shown in FIG. 6 is reset at a step S25, and then the program proceeds to a step S26.

At the step S26 are searched are a decremental value D_(T), and an incremental value D_(TRB). The decremental value D_(T) is determined depending on a change rate ΔP₂ of the supercharging pressure P₂, in accordance with a subroutine in FIG. 12. More specifically, if the throttle valve opening θ_(TH) is larger than the predetermined value θ_(THFB), the decremental value D_(T) is determined by the change rate ΔP₂ of supercharging pressure P₂ and the engine rotational speed N_(E) as shown in (a), (b), and (c) of FIG. 13. If θ_(TH) ≦θ_(THFB), D_(T) is set to 0%.

FIG. 13(a) shows a map of the decremental value D_(T) selected when the engine rotational speed N_(E) is equal to or lower than a predetermined first changeover engine rotational speed N_(FB1) (e.g. 3000 rpm), FIG. 13(b) shows a map of the decremental value D_(T) selected when the engine rotational speed N_(E) is above the first changeover engine rotational speed N_(FB1) and equal to or lower than a predetermined second changeover engine rotational speed NFB2 (e.g. 4500 rpm), and FIG. 13(c) shows a map of the decremental value D_(T) selected when the engine rotational speed N_(E) is above the second changeover engine rotational speed N_(FB2). The decremental value D_(T) is applied, as shown in FIG. 21, when the actual supercharging pressure P₂ becomes higher than a predetermined value P_(2ST) lower than a desired value P_(2REF) of supercharging pressure so that overshooting during rising of the supercharging pressure can be prevented. Further, D_(T) is set, as shown in FIG. 13 and as described above, in accordance with the engine rotational speed N_(E) and the change rate ΔP₂ of supercharging pressure. This is because the amount of overshooting depends on the engine rotational speed N_(E) and the change rate ΔP₂ of supercharging pressure when the predetermined value P_(2ST) is reached. D_(T) is set to a larger value as ΔP₂ is larger and as N_(E) is higher.

An incremental value D_(TRB) is determined in accordance with a subroutine shown in FIG. 14. More specifically, if the engine is in the open loop control mode, and at the same time the change rate ΔP₂ of supercharging pressure is negative, the incremental value D_(TRB) is determined by -ΔP₂ and the engine rotational speed N_(E) as shown in FIGS. 15(a), 15(b), and 15(c), and then the decremental value D_(T) is set to 0%. The incremental value D_(TRB) is set to 0% when ΔP₂ is positive, irrespective of whether the control is executed in feedback control mode or in open loop control mode. Similarly to the decremental value D_(T), the incremental value D_(TRB) is also changed as shown in FIG. 15 depending on the engine rotational speed N_(E) and the negative change rate -ΔP₂ of supercharging pressure. It is set to a larger value as N_(E) is higher and as |P₂ | is larger, whereby it is possible to carry out duty ratio control in a manner ensuring stable supercharging pressure P2 with almost no hunting in each operating region of the engine. In other words, according to the invention, for example, from the start of acceleration of the engine until the predetermined supercharging pressure value P_(2ST) is reached, the duty ratio D_(OUT) is set to and held at 100% to set the space area between the movable vanes 54 and the stationary vanes 49 to the minimum, to thereby increase the supercharging pressure P₂ at a high rate and hence enhance the accelerability of the engine. After the supercharging pressure P₂ has exceeded the predetermined value P_(2ST), the predetermined incremental value D_(TRB) is added to D_(M) so as to prevent hunting of supercharging pressure, which would otherwise occur in reaction to subtraction of the decremental value D_(T) for prevention of overshooting, whereby it is possible to carry out stable supercharging pressure control in each operating region of the engine.

After the correction coefficients K_(MODij), K_(PATC), K_(TATC) and K_(DOWN), the decremental value D_(T), and the incremental value D_(TRB) are thus determined, the program proceeds to a step S27.

At the step S27, the duty ratio D_(OUT) is calculated by the following equation:

    D.sub.OUT =K.sub.TATC ×K.sub.PATC ×K.sub.MODij ×K.sub.DOWN ×D.sub.M +D.sub.TRB -D.sub.T)

Thus, the duty ratio D_(OUT) outputted from the step S5 is set in accordance with operating conditions of the engine, by taking into account the external factors.

Further, at a step S28, the flag F is set to 1 to indicate that the engine is in the open loop control mode.

Then, at steps S29 and S30, it is determined whether or not the engine is in an operating condition in which the vehicle can run with the transmission set in a second speed position. More specifically, at the step S29, it is determined whether or not the engine rotational speed N_(E) falls within a range between a first predetermined value N_(SEC1), e.g. 4500 rpm, and a second value N_(SEC2), e.g. 6000 rpm, that is, N_(SEC1) ≦N_(E) ≦N_(SEC2). At the step S30, it is determined whether or not the vehicle speed V falls within a range between a first predetermined value V_(SEC1), e.g. 70 km/h, and a second predetermined value V_(SEC2;) e.g. 90 km/h, that is, V_(SEC1) ≦V≦V_(SEC2). If the answers to the questions of the steps S29 and S30 are both Yes, the duty ratio D_(OUT) is set to 0% at a step S31, followed by the program proceeding to a step S32. Thus, the supercharging pressure P₂ is decreased, whereby the ear shaft of the transmission, not shown, is prevented from being overloaded when it is in the second speed-holding position.

If N_(E) <N_(SEC1), N_(SEC2) <N_(E), V<V_(SEC1), or V_(SEC2) <V, the program proceeds to the step S32.

At the step S32, it is determined whether or not the automatic transmission is in the first speed position. If it is in the first speed position, the program proceeds to a step S33, while it is in a position other than the first speed position, the program proceeds to a step S37, where a timer t_(KDF) is reset, followed by the program proceeding to a step S38.

At the step S33, it is determined whether or not the automatic transmission was in the first speed position in the last loop. If the answer is yes, the program proceeds to a step S34, where it is determined whether or not the timer t_(KDF) has counted up a predetermined time period _(tDKFO) (e.g. 5 sec.). If t_(DKF) >t_(DKFO), the program proceeds to the step S38, while if t_(KDF) ≦t_(KDFO), the program proceeds to a step S36.

If it is determined at the step S33 that the automatic transmission was in a position other than the first speed position in the last loop, the timer t_(FBDLY) is reset at a step S35, followed by the program proceeding to the step S36 where the duty ratio D_(OUT) is set to 0%, and then to the step S38.

The steps S32-S37 are provided for the kicking-down operation. That is, by virtue of the steps, when kicking-down is made from a position other than the first speed position to the first speed position, and until a predetermined time period, e.g. 5 sec., elapses after the kicking-down, the duty ratio D_(OUT) is maintained at 0%, thereby preventing overload on the first speed gear.

At the step S38, the duty ratio D_(OUT) is checked to make sure that it is within a predetermined range defined by upper and lower limit values. The upper and lower limit values of D_(OUT) are set in accordance with the engine rotational speed N_(E). If D_(OUT) is within the predetermined range, it is outputted at the step S5.

If it is determined at the step S24 that θ_(TH) >θ_(THFB), the program proceeds to a step S39 where it is determined whether or not the flag F assumed 1 in the last loop, i.e. whether or not the engine was in the open loop control mode in the last loop. If F=1, it is determined at a step S40 whether or not the supercharging pressure P₂ is above the duty ratio control-starting value P_(2ST). The duty ratio control-starting value P_(2ST) is obtained by the equation P_(2ST) =P_(2REF) -ΔP_(2ST). ΔP_(2ST) is set depending on the engine rotational speed N_(E), as shown in FIGS. 16(a), 16(b) and 16(c). Here, similarly to the above-described D_(T) and D_(TRB), ΔP_(2ST) is set in accordance with the engine rotational speed N_(E) and the change rate ΔP₂ of supercharging pressure to ensure the optimum duty control. It is set to a larger value as the engine rotational speed N_(E) is higher and as the change rate ΔP₂ of supercharging pressure is larger.

If P₂ >P_(2ST) at the step S40, it is determined at a step S41 whether or not the supercharging pressure P₂ is above the feedback control-starting value P_(2FB). The feedback control-starting supercharging pressure P_(2FB) is obtained by the equation P_(2FB) =P_(2REF) -ΔP_(2FB). As shown in FIGS. 17(a), 17(b) and 17(c), ΔP_(2FB) is set depending on the engine rotational speed N_(E). Similarly to the above-described ΔP_(2ST), D_(T), and D_(TRB), ΔP_(2FB) is determined in accordance with the engine rotational speed N_(E) and the change rate ΔP₂ of supercharging pressure to ensure the optimum duty ratio control. It is set to a larger value as the engine rotational speed N_(E) is higher and as the change rate ΔP₂ of supercharging pressure is larger. If P₂ >P_(2FB) at the step S41, the program proceeds to a step S42.

At the step S42, it is determined whether or not the delaying time period t_(FBDLY) has elapsed. If the delaying time period t_(FBDLY) has elapsed, the program proceeds to a step S43. In the meanwhile, if F=0 at the step S39, the program skips over the steps S40 to S42 to the step S43, if P₂ ≦P_(2ST) at the step S40, the program proceeds to the step S44, if P₂ ≦P_(2FB) at the step S41, the program proceeds to the step S25, and if the delaying time period t_(FBDLY) has not elapsed at the step S42, the program proceeds to the step S26.

At the step S44, a predetermined basic duty ratio value D_(SCRB) as a second supercharging pressure control amount is searched which is dependent on the engine rotational speed N_(E), as shown in FIG. 18. The basic duty ratio value D_(SCRB) searched is substituted into the following equation to calculate the duty ratio D_(OUT) :

    D.sub.OUT =D.sub.SCRB ×K.sub.TATC ×K.sub.PATC

Subsequently, at a step S46, a timer t_(FBDLY) is reset, followed by the program proceeding to the step S38.

The steps S44 and S45 are for achieving stable supercharging pressure control in an operating region in which the supercharging pressure P₂ is below the value P_(2ST). That is, since the duty ratio D_(OUT) is determined based upon the predetermined value D_(SCRB) corresponding to the engine rotational speed N_(E), overshooting can be prevented without fail, irrespective of the change rate ΔP₂ of the supercharging pressure P₂. Alternatively, at the steps S44 and S45, the duty ratio D_(OUT) may be set to 0% to simplify the process.

As described above, according to the control based on the duty ratio control-starting value P_(2ST) and the feedback control-starting value P_(2FB), the supercharging pressure P₂ is controlled in feedback control if P₂ >P_(2FB), in starting mode at the step S44 et seq if P₂ ≦P_(2ST), and based on the basic duty ratio DM if P_(2ST) <P₂ <P_(2FB), respectively.

However, since the supercharging pressure P₂ varies depending on the ambient air pressure (atmospheric pressure), if the value P_(2ST) and P_(2FB) are determined only depending on the engine rotational speed N_(E) and the supercharging pressure change rate ΔP₂, the starting control will be often executed, while the feedback mode control will not be executed as expected. To avoid this, the values P_(2ST) and P_(2FB) are corrected by the ambient air pressure.

At the step S43, it is determined whether or not the absolute value of change rate ΔP₂ of supercharging pressure is above a predetermined supercharging pressure difference G_(dP2) for determining whether to start the feedback control. The supercharging pressure difference G_(dP2) is set, for example, at a value of 30 mmHg. If the absolute value of ΔP₂ is above the value G_(dP2), the program returns to the step S26, and if the absolute value of ΔP₂ is equal to or lower than the value G_(dP2), the program proceeds to a step S47. If the feedback control is started when |ΔP₂ >G_(dP2), it may result in hunting. Therefore, the program returns to the step S26 to carry out the open loop control. As described above, in the open loop control, correction of the basic dutV ratio D_(M) by D_(T) and D_(TRB) is carried out to prevent hunting and overshooting of supercharging pressure. Therefore, the step S47 is provided mainly for the fail-safe purpose.

The feedback control is started at the step S47, where the desired supercharging pressure P_(2REF) is determined depending on the engine rotational speed N_(E) and the intake air temperature T_(A). The feedback control is started on condition that θ_(TH) >θ_(THFB) at the step S24. Under this condition, the desired supercharging pressure P_(2REF) is determined by the use of the engine rotational speed N_(E) and the intake air temperature T_(A) as parameters enabling accurate determination of operating conditions of the engine. If θ_(TH) >θ_(THFB), under a medium or high load operating condition, the engine rotational speed N_(E) and the throttle valve opening θ_(TH) behave approximately in the same manner. Therefore, the N_(E) can be an effective parameter representing operating conditions of the engine. In the meanwhile, the intake air temperature T_(A) is the temperature of intake air downstream of the intercooler 4 as shown in FIG. 2, and therefore can be a parameter accurately representing the condition of intake air introduced into the combustion chambers. Therefore, it is possible to set the desired supercharging pressure P_(2REF) to values exactly responsive to operating conditions of the engine by the use of a map determined by the engine rotational speed N_(E) and the intake air temperature T_(A). The desired supercharging pressure P_(2REF) is set to a lower value as the intake air temperature T_(A) is decreased. More specifically, the increase rate of the supercharging pressure tends to be larger when the intake air temperature T_(A) is lower. Therefore, the desired supercharging pressure is set in the above manner, whereby the minimum opening control can be terminated at appropriate timing, and hence the accelerability can be further improved.

At a step S48, it is determined whether or not the automatic transmission is in the first speed position. If the automatic transmission is in the first speed position, calculation of P_(2REF) =P_(2REF) -ΔP_(2REFF) is carried out at a step S49 in accordance with the subroutine shown in FIG. 8 when the operating condition of the engine is within the predetermined operating zone shown by hatching in FIG. 9, and then the program proceeds to a step S51. ΔP_(2REFF) is a predetermined decremental value which is applied when the transmission is in the first speed position. If it is determined at the step S48 that the transmission is in a position other than the first speed position, calculation of P_(2REF) =P_(2REF) -ΔP_(2REFOS) is carried out at a step S50 in accordance with the subroutine shown in FIG. 10, and then the program proceeds to the step S51. ΔP_(2REFOS) is a predetermined decremental value which is applied when the transmission is in a position other than the first speed position.

At the step S51, an atmospheric pressure-dependent correction coefficient K_(PAP2) for correcting the supercharging pressure is determined in accordance with the atmospheric pressure P_(A), and then at a step S52, the following calculation is carried out:

    P.sub.2REF =P.sub.2REF ×K.sub.PAP2 ×K.sub.REFTB

where K_(REFTB) is a correction coefficient responsive to a knocking condition of the engine.

At a step S53, it is determined whether the absolute value of the difference between the desired supercharging pressure P_(2REF) and the supercharging pressure P₂ detected in the present loop is equal to or higher than a predetermined value G_(P2). The predetermined value GP₂ is a value defining the insensitive pressure width in the feedback control mode, and is set, for example, at 20 mmHg. If the absolute value of the difference between the desired supercharging pressure and the actual supercharging pressure is equal to or higher than the predetermined value G_(P2), the program proceeds to a step S54, and if not, the program proceeds to a step S61.

At the step S54, a proportional control term D_(P) for correcting the duty ratio is calculated by the following equation:

    D.sub.P =K.sub.P ×(P.sub.2REF -P.sub.2)

where K_(P) is a feedback coefficient for the proportional control term, and is obtained in accordance with a subroutine shown in FIG. 19. In FIG. 19, if the engine rotational speed N_(E) is equal to or lower than the first changeover engine rotational speed N_(FB1), K_(P1) is obtained and at the same time a feedback coefficient K_(I1) for an integral control term, described later, is obtained. If the engine rotational speed N_(E) is above the first changeover engine rotational speed N_(FB1) and equal to or lower than the second changeover engine rotational speed N_(FB2), K_(P2) and K_(PI2) are obtained. If the engine rotational speed N_(E) is above the second changeover engine rotational speed N_(FB2), K_(P3) and K_(PI3) are obtained.

At a step S55, the correction coefficient K_(MODij) is determined in accordance with the engine rotational speed N_(E) and the intake air temperature T_(A). At a step S56, it is determined whether or not the flag F assumed 1 in the last loop, i.e whether or not the present loop is the first loop in which the feedback control mode has been started. If F=1, an integral control term D_(I)(n-1) applied in the last loop is obtained at a step S57 by the following equation:

    D.sub.I(n-1) =K.sub.TATC ×K.sub.PATC ×D.sub.M ×(K.sub.MODij -1)

After this calculation, the program proceeds to a step S58. If F=0 at the step S56, the program skips over the step S57 to the step S58.

At the step S58, an integral control term D_(In) for the present loop is calculated by the following equation:

    D.sub.In =D.sub.I(n-1) +K.sub.I +(P.sub.2REF -P.sub.2)

where K_(I) represents feedback coefficients K_(I1) -K_(I3) obtained when the feedback coefficient K_(P) is calculated at the step S54 in accordance with the subroutine in FIG. 19.

Then the program proceeds to a step S59, where the duty ratio D_(OUT) is calculated by the following equation:

    D.sub.OUT =K.sub.TATC ×K.sub.PATC ×K.sub.DOWN ×D.sub.M +D.sub.P +D.sub.In

Then, at a step S60, the flag F is set to 0, and the program proceeds to the step S38.

If it is determined at the step S53 that the absolute value of the difference between the desired supercharging pressure P_(2REF) and the actual supercharging pressure P₂ is smaller than the predetermined value G_(P2), D_(P) is set to 0 and D_(In) is set to D_(I)(n-1) at a step S61. Then at steps S62 to S66, it is determined whether or not the atmospheric pressure P_(A) is above a predetermined value P_(AMOD) (e.g 650 mmHg), whether or not the engine coolant temperature T_(W) is within a predetermined range, i.e. above T_(WMODL) and below T_(WMODH), whether or not a retarding amount T_(ZRET) is 0, i.e. whether or not the engine is not under a knocking condition, whether or not the transmission is in a position other than the first speed position, and whether or not K_(REFTB) responsive to the knocking condition is equal to or lower than 1.0. If all these conditions are satisfied, the program proceeds to a step S67, and if any one of them is not satisfied, the program proceeds to the step S59. More specifically, if all the conditions of the steps S62 to S66 are satisfied, the correction coefficient K_(MODij) is learned and stored at the steps S67 and S70, whereas if any of the conditions of the steps S62 to S66 is not satisfied, the program jumps to the step S59 without learning the correction coefficient K_(MODij). Thus, it is possible to prevent the correction coefficient K_(MODij) from being deviated from a proper value, and hence control supercharging pressure to a more suitable value in the open loop control mode.

At the step S67, a coefficient K_(R) for learning the correction coefficient K_(MODij) for duty ratio is calculated by the following equation:

    K.sub.R =(K.sub.TATC ×D.sub.M +D.sub.In)/(K.sub.TATC ×D.sub.M)

At a step S68, in order to determine and learn the correction coefficient K_(MODij), the following calculation is carried out:

    K.sub.MODij =(C.sub.MOD ×K.sub.R)/65536+[(65536-C.sub.MOD)×K.sub.MODij)]/65536

where C_(MOD) represents a variable set to a suitable value selected from 1-65536 experimentally depending on the characteristics of the supercharging pressure control system, the engine etc..

At a step S69, K_(MODij) obtained at the step S68 is subjected to limit checking. Thereafter, at the step S7O K_(MODij) is stored in a back-up RAM, not shown, following by the program proceeding to the step S59.

According to the above-described control of the duty ratio of the solenoid 70 of the electromagnetic control valve 69, under the condition that the automatic transmission is in the first speed position, if the engine is in the open loop control mode, D_(F) is subtracted from the basic duty ratio D_(M) at the step S20 when the operating condition of the engine is in the predetermined operating zone shown in FIG. 9, and if the engine is in the feedback control mode, ΔP_(2REFF) is subtracted from the desired supercharging pressure P_(2REF) at the step S49 when the operating condition of the engine is in the predetermined operating zone. Thus, excessive load on the automatic transmission due to sudden start of the vehicle and overload on the engine under the condition that the automatic transmission is in the first speed position can be prevented by decreasing the supercharging pressure through subtraction from the basic duty ratio D_(M). Further, even if the control mode is shifted from the open loop control mode to the feedback control mode when the transmission is in the first speed position, occurrence of hunting in the transitional state can be prevented since subtraction from the desired supercharging pressure P_(2REF) is carried out.

Suppose that the gear position of the transmission is shifted as shown in the lower part of FIG. 20. As known, when the gear position of the transmission is shifted, the engine rotational speed N_(E) is decreased. However, there is a time lag before the actuator 60 starts to operate in response to a signal from the control unit C. Therefore, the supercharging pressure P₂ does not properly correspond to the change in the engine rotational speed N_(E) and overshooting of the supercharging pressure may arise. As shown by the broken line in FIG. 20, when the gear position of the transmission is shifted immediately after acceleration in a medium or high engine speed range, the supercharging pressure may exceed the upper limit value P_(2HG). However, in the embodiment of FIGS. 5A and 5B at the step S2I and at the step S50, subtraction from the basic duty ratio D_(M) and subtraction from the desired supercharging pressure P_(2REF) are carried out, respectively, in accordance with the subroutine shown in FIG. 10. More specifically, when the gear position of the transmission is shifted, under the conditions that the throttle valve opening θ_(TH) is above the predetermined value θ_(THOS), the engine rotational speed N_(E) is above the predetermined value N_(EOS), and the intake pressure P_(B) is above the predetermined value P_(BOS), i.e. in the medium or high speed range, D_(OS) is subtracted from the basic duty ratio D_(M) in the open loop control mode depending on the change rate ΔP₂ of supercharging pressure P₂, and ΔP_(2REFOS) is subtracted from the desired supercharging pressure P_(2REF) in the feedback control mode. Thus, as shown by the solid line in FIG. 20, overshooting at the time of shifting of the transmission position is greatly reduced, whereby it is possible to prevent hunting and carry out stable supercharging pressure control.

Further, when the control mode is shifted from the open loop control mode to the feedback control mode, as shown in FIG. 21, a drop in the supercharging pressure P₂ is prevented whereby the control mode can be smoothly shifted to the feedback control mode. More specifically, at the start of the engine, the duty ratio D_(OUT) is set to 0%, and in the open control mode in which the throttle valve opening θ_(TH) is below the predetermined value θ_(THFB), D_(T) is set to 0% at the step S26 in accordance with the subroutine shown in FIG. 12. As stated before, the control mode starts to shift from the open loop control mode to the feedback control mode when the throttle valve opening θ_(TH) has exceeded the predetermined value θ_(THFB). When the supercharging pressure P₂ has exceeded P_(2ST) and the throttle valve opening θ_(TH) is above the predetermined value θ_(THFB), the subtraction of D_(M) =D_(M) -D_(T) is carried out to prevent overshooting of the supercharging pressure.

In some cases, if D_(T) alone is thus subtracted from the basic duty ratio D_(M), the supercharging pressure P₂ may drop as shown by the broken line in FIG. 21, in relation to the subtraction. However, according to the subroutine of FIG. 14, if ΔP₂ ≦0, D_(T) is set to 0%, and only D_(TRB) is added to the basic duty ratio D_(M). Therefore, it is possible to cope with the possible drop in the supercharging pressure P₂ to thereby smoothly shift the control mode to the feedback control mode while preventing occurrence of hunting of the supercharging pressure.

The aforesaid control of duty ratio of the solenoid 70 of the electromagnetic control valve 69 is carried out when the electromagnetic valve 72 is closed. If the electromagnetic valve 72 is opened, intake pressure P_(B) is introduced into the second pressure chamber 63 of the actuator 60, which in turn causes the movable vanes 54 of the variable capacity turbocharger 5 to operate such that the space area between the movable and stationary vanes 54, 49 is increased.

In this manner, in addition to the control of operation of the electromagnetic control valve 69 for introducing supercharging pressure P₂ into the first pressure chamber 62 of the actuator 60 in accordance with the main routine shown in FIGS. 5A and 5B, intake pressure P_(B) is introduced into the second pressure chamber 63 of the actuator 60 by way of the electromagnetic control valve T2, and at the same time the electromagnetic valve 72 is controlled based on the intake pressure P_(B) from the intake pressure sensor S_(PB), which makes it possible to carry out more accurate control of the supercharging pressure. The reason for this is as follows. Since the supercharging pressure P₂ is detected between the variable capacity turbocharger 5 and the intercooler 4, it is impossible to detect a subtle operation of the throttle valve 74. In contrast, since the intake pressure P_(B) is detected downstream of the throttle valve 74, it is possible to detect a subtle operation thereof. Thus, by the use of both the supercharging pressure sensor S_(P2) positively sensitive to the operation of the turbocharger 5 and the intake pressure sensor S_(PB) positively sensitive to the operation of the throttle valve 74, the operation of the whole intake system including the turbocharger 5 can be more accurately reflected upon the control of the supercharging pressure.

Next, with reference to FIG. 22, the manner of control of controlling the solenoid 73 of the electromagnetic valve 72 by the control unit C will be described below.

At a step L1, it is determined whether or not a predetermined time period, e.g. 2 minutes, has elapsed from the start of the engine. If the predetermined time period has not elapsed, the program proceeds to a step L2, where the solenoid 73 is energized, whereby the actuator 60 is operated to cause the movable vanes 49 to operate such that the space area between the movable and stationary vanes 54, 49 is increased. This can cope with the start of the engine in cold weather. Thus, excessive supercharging under cold weather is prevented, and the catalyst temperature can be gently raised. If the predetermined time period has elapsed at the step L1, the program proceeds to a step L3, where it is determined whether or not the speed V of the vehicle is above a predetermined value V_(OP3), which is provided with a hysteresis between when the vehicle speed V increases and when it decreases and is set to, for example, 90/87 km/h. If V>V_(OP3), the program proceeds to a step L4, whereas if V≦V_(OP3), the program proceeds to a step L5.

At the step L4, it is determined whether or not the throttle valve opening change rate Δθ_(TH) is below a predetermined value Δθ_(THOP2). The predetermined Δθ_(THOP2) is provided with a hysteresis similar to that of the vehicle speed V_(OP3). If Δθ_(TH) <Δθ_(THOP2), the program proceeds to a step L2, and otherwise, the program proceeds to the step L5.

At the step L5, it is determined whether or not the vehicle speed V is below a predetermined value V_(OP1). The predetermined value V_(OP1) also has a hysteresis and is set to, for example, 65/63 km/h. If V<V_(OP1), the program proceeds to a step L7, whereas if V≧V_(OP1), the program proceeds to a step L6, where the solenoid 73 is deenergized. At the step L7, it is determined whether or not the vehicle speed V is above a predetermined value V_(OP2). The predetermined value V_(OP2) also has a hysteresis, and is set to, for example, 4/3 km/h. If V>V_(OP2), the program proceeds to a step L12, whereas if V≦V_(OP2), the program proceeds to a step L8.

At the step L8, it is determined whether or not the vehicle speed V detected in the last loop is above the predetermined value V_(OP2). If V>V_(OP2), the program proceeds to a step L9, where the t_(OP) timer for counting a time period t_(OP) is reset, and then the program proceeds to a step L10. If V≦V_(OP2), the program directly proceeds to the step L10. At the step L10, it is determined whether or not the solenoid 73 was energized in the last loop. If the solenoid 73 was deenergized in the last loop, the program proceeds to the step L6, whereas if it was energized in the last loop, the program proceeds to a step L11, where it is determined whether or not the time period t_(OP) exceeds a predetermined time period t_(OPO). If t_(OP) >t_(OPO), the program proceeds to the step L6, whereas if t_(OP) ≦t_(OPO), the program proceeds to the step L2.

At the step L12, it is determined whether or not the engine rotational speed N_(E) is below a predetermined value N_(EOP). The predetermined value N_(EOP) has a hysteresis, and is set to, for example, 2500/2300 rpm. If N_(E) ≧N_(EOP), the program proceeds to the step L6, whereas if N_(E) <N_(EOP), the program proceeds to a step L13.

At the step L13, it is determined whether or not the intake pressure P_(B) is below a predetermined value P_(BOP). The predetermined value P_(BOP) has a hysteresis, and is set to, for example, -100/-150 mmHg. If P_(B) ≦P_(BOP), the program proceeds to the step L6, whereas if PB<P_(BOP), the program proceeds to a step L14.

At the step LI4, it is determined whether or not the throttle valve opening θ_(TH) is below a predetermined value θ_(THOP). The predetermined value θ_(THOP) is set at 20/15 degrees. If θ_(TH) ≧θ_(THOP), the program proceeds to the step L6, whereas if θ_(TH) <θ_(THOP), the program proceeds to a step L15.

At the step L15, it is determined whether or not the throttle valve opening change rate Δθ_(TH) is positive and at the same time below a predetermined value Δθ_(THOP1) which is set such that it has a hysteresis. If θ<Δθ_(TH) <Δθ_(THOP1), the program proceeds to the step L2, and otherwise, the program proceeds to the step L6.

According to the above-described control manner, if it is determined at the steps L3 and L4 that the vehicle speed V is higher than 90/87 km/h, and that the acceleration thereof is gentle as shown by 0<ΔθTH<ΔθTHOP2, the movable vanes 54 of the turbocharger 5 are operated such that the space area between the movable vanes 54 and the stationary vanes 49 is increased, whereby pumping loss can be prevented. In other words, when the vehicle is cruising at a high speed, acceleration of the engine is not required, and if the movable vanes 54 are operated such that the supercharging pressure is increased, pumping loss may occur due to rise in the back pressure in the exhaust manifold resulting from a high engine rotational speed.

If it is determined at the step L5 that the vehicle is running at a speed higher than 65/63 km/h, the solenoid 73 is deenergized. This is because when the vehicle is running at such a high speed, the supercharging pressure can be sufficiently controlled by the electromagnetic control valve 69 in accordance with the routine shown in FIGS. 5A and 5B. Further, at the steps L7 to L11, if the vehicle is running at a speed lower than 4 or 3 km/h, i.e. it is almost stationary, and at the same time if the vehicle was almost stationary in the last loop, the t_(OP) timer is reset, and then until the time period, for example, one minute, has elapsed, the solenoid 73 is energized so as to operate the movable vanes 54 such that the space area between the movable and stationary vanes 54, 49 is increased. If the movable vanes 54 are in such a position as to make the space area narrower at the restart of the vehicle, the supercharging pressure P₂ is temporarily increased to apply excessive load on the starting gear etc. Therefore the solenoid 73 is energized to prevent such application of the excessive load on the starting gear etc. Further, if the movable vanes 54 are in such a position as to make the space area narrower when the vehicle is running at a speed lower than 4 or 3 km/h, rotation of the variable capacity turbocharger 5 by inertia etc is promoted. On this occasion, the throttle valve opening θ_(TH) is almost fully closed, and therefore the supercharging pressure increases the pressure within the intake pipe on the upstream side of the throttle valve to cause surging of the latter pressure. Therefore, the movable vanes 54 are operated such that the space area is increased, to prevent surging of the intake pipe pressure. In addition, the control of supercharging pressure carried out at the steps L7 to L11 contributes to rise in the catalyst temperature immediately after the start of the vehicle when the weather is cold.

If at the steps L12 to L15, all the conditions of V_(OP2) <V<V_(OP1), N_(E) <N_(EOP), P_(B) <P_(BOP), θ_(TH) <θ_(THOP), and 0<Δθ_(TH) <Δθ_(THOP1) are satisfied, i.e. if the vehicle is gently accelerated under partial load as in the 10 mode running, the solenoid 73 is energized to decrease the supercharging pressure P2, whereby pumping loss can be prevented.

Next, a variation of the manner of control of the solenoid 73 of FIG. 22 will now be described with reference to FIG. 23.

At a step M1, it is determined whether or not the engine is in the starting mode, that is, whether or not the engine is in a cranking condition. If the engine is in the starting mode, a flag F_(S) is set to 0 at a step M2, followed by the program proceeding to a step M3 where the solenoid 73 is deenergized. By deenergizing the solenoid 73, the electromagnetic valve 72 is closed to interrupt the introduction of the intake pressure P_(B) into the second pressure chamber 63 of the actuator 60. In this state, the actuator 60 and hence the movable vanes 54 are controlled by the supercharging pressure P₂ introduced into the first pressure chamber 62 by the electromagnetic control valve 69. On the contrary, if the solenoid 73 is energized, the electromagnetic valve 72 is opened to introduce the intake pressure P_(B) into the second pressure chamber 63, whereby the actuator 60 drives the movable vanes 54 to reduce the supercharging pressure P₂. The flag F_(S) at the step M2 is used to determine whether to allow the energization of the solenoid 73. If F_(S) =0, the solenoid 73 is not energized.

If it is determined at the step M1 that the engine is not in the starting mode, the program proceeds to a step M4, where it is determined whether or not the TDC signal pulse inputted is the first pulse in the basic mode, which means that the present loop is the first loop. If the present loop is the first loop, the flag F_(S) is set to 1 at a step M5, followed by the program proceeding to a step M6, whereas if the present loop is not the first loop, the program skips over the step M5 to the step M6.

At the step M6, it is determined whether or not the intake air temperature T_(A) downstream of the intercooler 4 is below a predetermined value T_(AOPO), e.g. -15° C. If T_(A) <T_(AOPO), the program proceeds to a step M7, where it is determined whether or not the engine rotational speed N_(E) is above a predetermined value N_(OP1), e.g 3500 rpm. If N_(E) >N_(OP1), the solenoid 73 is energized at a step M8, while if N_(E) ≦±N_(OP1), the solenoid 73 is deenergized at the step M3. That is, if T_(A) <T_(AOPO) and at the same time N_(E) >N_(OP1), the solenoid 73 is energized to reduce the supercharging pressure P₂.

If T_(A) ≧T_(AOPO) at the step M6, the program proceeds to a step M9, where it is determined whether or not a predetermined time period (e.g. 2 min.) has elapsed after the start of the engine. If the predetermined time period has not elapsed, the program proceeds to a step M10, where it is determined whether or not the engine rotational speed N_(E) is below a predetermined value N_(OP2), e.g. 3000 rpm. If N_(E) <N_(OP2), the program proceeds to a step M11, while if N_(E) ≧N_(OP2), the program proceeds to the step M2. At the step M11, it is determined whether or not the change rate Δθ_(TH) of the throttle valve opening θ_(H) is within a predetermined range, that is, 0<Δθ_(TH) <Δθ_(THOP2). If 0<Δθ_(THOP2), the program proceeds to the step M8, where the solenoid 73 is energized, whereas if the condition is not fulfilled, the program proceeds to the step M2. This means that when the intake air temperature T_(A) is above the predetermined value T_(AOPO), and at the same time the predetermined time period has not elapsed after the start of the engine, the solenoid 73 is deenergized, if the condition of N_(E) ≧N_(OP2) is fulfilled and at the same time the condition of 0<Δθ_(TH) <Δθ_(THOP2) is not fulfilled, whereby accurate control of the supercharging pressure can be effected even before the predetermined time period elapses after the start of the engine. On the other hand, if N_(E) <N_(OP2) and at the same time 0<Δθ_(TH) <Δθ_(THOP2), the solenoid 73 is energized so that the actuator 60 drives the movable vanes 54 to move in such a direction as to increase the space area defined between the movable vanes 54 and the stationary vanes 49. This improves the startability of the engine in cold weather, by inhibiting supercharging at cold starting of the engine. Further, the temperature of the catalyst can be gradually increased.

At the step M9, if it is determined that the predetermined time period has elapsed, the program proceeds to a step M12, where it is determined whether or not the vehicle speed V is below the predetermined value V_(OP1). If V<V_(OP1), the program proceeds to a step M13, while if V≧V_(OP1), the program proceeds to the step M2 to deenergize the solenoid 73. At the step M13, it is determined whether or not the vehicle speed V is below the predetermined value V_(OP2). If V>V_(OP2), the program proceeds to a step M14, while if V≦V_(OP2), the program proceeds to a step M19.

At the step M14, it is determined whether or not the vehicle speed V detected in the last loop is above the predetermined value V_(OP2). If V>V_(OP2), a timer T_(OP) is reset at a step M15, followed by the program proceeding to a step M16, while if V≦V_(OP2), the program proceeds to the step M16. At the step M16, it is determined whether or not the solenoid 73 was energized in the last loop. If it was deenergized, the program proceeds to the step M3, while if it was energized, the program proceeds to a step M17, where it is determined whether or not the timer t_(PO) has counted up a predetermined value t_(OPO). If t_(PO) >t_(OPO), the program proceeds to a step M18, while if t_(PO) ≦t_(OP), the program proceeds to the step M8. At the step M18, it is determined whether or not the engine rotational speed N_(E) is above a predetermined value N_(OP4) (e.g. 1200 rpm). If N_(E) >N_(OP4), the program proceeds to the step M8, while if N_(E) ≦N_(OP4), the program proceeds to the step M3.

The step M18 is provided for the following reason:

Even when the vehicle speed is below the predetermined value V_(OP2), if the engine is in a fast idling condition or a like condition where the engine rotational speed N_(E) is above N_(OP4), then the driver does not want supercharging, and if in such condition the movable vanes 54 are moved to reduce the space area between the vanes 54 and 49, the flow resistance of exhaust gases flowing through the spaces will increase to badly affect the combustion efficiency of the engine and increase the fuel consumption due to unnecessarily increased engine output. Therefore, in such a condition the solenoid 73 is energized.

At the step M19, it is determined whether or not the engine rotational speed N_(E) is below a predetermined value N_(OP3). The predetermined value N_(OP3) has a hysteresis and is set to e.g. 2500/2300 rpm between when N_(E) increases and when N_(E) decreases. If N_(E) ≧N_(OP3), the program proceeds to the step M3, while if N_(E) <N_(OP3), the program proceeds to a step M20.

At the step M20, it is determined whether or not the intake pressure P_(B) is below the predetermined value P_(BOP). If P_(B) ≧P_(BOP), the program proceeds to the step M2, while if P_(B) <P_(BOP), the program proceeds to a step M21.

At the step M21, it is determined whether or not the throttle valve opening θ_(TH) is below the predetermined value θ_(THOP). If θ_(TH) ≧θ_(THOP), the program proceeds to the step M2, while if θ_(TH) <θ_(THOP), the program proceeds to a step M22.

At the step M22, it is determined whether or not the change rate Δθ_(TH) of the throttle valve opening θ_(TH) is below the predetermined value Δθ_(THOP1). If Δθ_(TH) <Δθ_(THOP1), the program proceeds to a step M23, while if Δθ_(TH) ≧Δθ_(THOP1), the program proceeds to the step M2. At the step M23, it is determined whether or not the flag F_(S) is 0. If F_(S) =0, the solenoid 73 is deenergized at the step M3, while F_(S) =1, the solenoid 73 is energized at the step M8.

As described above, at the steps M6 and M7, if the intake air temperature T_(A) is below the predetermined value T_(AOPO) and at the same time the engine rotational speed N_(E) is above the predetermined value N_(OP1), the solenoid 73 is energized to drive the movable vanes 54 to move in the direction as to increase the space area between the movable vanes 54 and the stationary vanes 49. Therefore, the supercharging pressure can be increased at the start of the engine, and simultaneously, overload on the engine can be avoided when the intake air temperature T_(A) is too low.

FIG. 24 shows a program for controlling the electromagnetic control valve 69, according to a second embodiment of the invention. The second embodiment is distinguished from the first embodiment of FIGS. 5A and 5B, in that instead of using the supercharging pressure sensor S_(P2), the supercharging pressure control is effected based upon the intake pressure P_(B) detected by the intake pressure sensor S_(PB). This is based on the fact that the feedback control of the supercharging pressure is effected in an operating condition of the engine where the throttle valve 74 is almost fully open, in which condition information relating to the supercharging pressure can be obtained by the intake pressure P_(B).

At a step S101, the basic duty ratio D_(M) is read from a D_(M) map in response to the throttle valve opening θ_(TH) and the engine rotational speed N_(E). FIG. 25 shows an example of the D_(M) map in which the throttle valve opening θ_(TH) is classified into sixteen predetermined values θ_(THV) -θ_(THV16) within a predetermined range, while the engine rotational speed N_(E) is classified into twenty predetermined values N_(V1) -N_(V20). The basic duty ratio D_(M) is determined by means of interpolation, if θ_(TH) or N_(E) falls between respective adjacent predetermined values. By setting the basic duty ratio D_(M) by the use of the D_(M) map, the duty ratio D_(OUT) of the electromagnetic control valve 69 can be controlled more accurately in response to operating conditions of the engine E.

Next it is determined at a step S102 whether or not the gear position of the transmission is in a first speed position. This determination is carried out in accordance with a subroutine, e.g. shown in FIG. 26. In the subroutine, it is determined whether or not the speed V of the vehicle is lower than a predetermined value V_(L) which is normally obtained in the first speed position. If V<V_(L), it is then determined whether or not the vehicle speed V is smaller than a predetermined value V_(F) corresponding to the engine rotational speed N_(E). If V≧V_(L) or V≧V_(F), it is determined that the gear position is not in the first speed position, whereas if V<V_(L) and at the same time V<V_(F), it is determined that the gear position is in the first speed position.

FIG. 27 shows a table for determining the predetermined value V_(F). When the transmission is in the first speed position, the ratio between the engine rotational speed N_(E) and the vehicle speed V is constant. The table is set so as to satisfy this constant ratio relationship and provided with predetermined values N_(F1) -N_(F9) of the engine rotational speed and predetermined values V_(F1) -V_(F8) of the vehicle speed V. It is determined that the transmission is in the first speed position when the vehicle speed V is lower than the predetermined value V_(F) corresponding to the actual engine rotational speed N_(E). By virtue of the above determinations, it is possible to determine without a gear position sensor or the like whether or not the transmission is in the first speed position, irrespective of whether the transmission is manual or automatic.

Referring again to FIG. 24, if it is determined at the step S102 that the transmission is in the first speed position, then at a step S103 the basic duty ratio D_(M) determined at the step S101 is decreased by subtracting a predetermined value D_(F) from the basic duty a ratio D_(M), followed by the program proceeding to a step S104. On the other hand, if the transmission is in a position other than the first speed position, the program jumps to the step S104. In this way, the basic duty ratio D_(M) is set to a value smaller by the predetermined value D_(F) when the transmission is in the first speed position than when it is not in another position. By virtue of this control, when the transmission is in the first speed position, the supercharging pressure is moderately suppressed as a whole so that an abrupt increase or overboosting in the supercharging pressure can be prevented, as indicated by the solid line in FIG. 39. Furthermore, when the transmission is in a position other than the first speed position, the supercharging pressure can be controlled to a sufficiently high value, thereby enabling to attain desired accelerability, as indicated by the broken line in FIG. 39.

At the step S104, an intake air temperature-correcting coefficient K_(TATC) is read from a K_(TATC) map in response to the engine rotational speed N_(E) and the intake air temperature T_(A). FIG. 28 shows an example of the K_(TATC) map, in which the engine rotational speed N_(E) is classified into twenty predetermined values N_(V1) -N_(V20) within a predetermined range, similarly to the D_(M) map, while the intake air temperature T_(A) is classified into eight predetermined values T_(AV1) -T_(AV8). By virtue of the K_(TATC) map, the intake air temperature-correcting coefficient K_(TATC) is set to a suitable value.

Then at a step S105, the change rate ΔP_(B) of the intake air pressure P_(B), hereinafter merely called "the change rate", is calculated by subtracting a value PB_(n-3) detected in the third loop before the present loop from a value P_(Bn) detected in the present loop. The change rate ΔP_(B) is applied to setting of constants used for calculating the duty ratio D_(OUT), as hereinafter described in detail, whereby the increase rate of the supercharging pressure is controlled to a desired value.

Next, at a step S106, it is determined whether or not the supercharging pressure is in a range in which open loop control is to be effected. This determination is carried out in accordance with a subroutine shown in FIG. 29.

First, at a step S201 of the FIG. 29 subroutine, it is determined whether or not the throttle valve opening θ_(TH) is larger than a predetermined value θ_(THFB) indicating that the throttle valve 74 is almost full open. If θ_(TH) ≦θ_(THFB), that is, if the throttle valve 74 is not almost fully open, it is determined that the open loop control should be effected, followed by the program proceeding to a step S216 et seq, hereinafter referred to. That is, feedback control is effected only when the throttle valve 74 is almost fully open.

If it is determined at the step S201 that θ_(TH) >θ_(THFB), it is determined at a step S202 whether or not, a flag F set in the last loop at a step S203 or S221, hereinafter referred to, is equal to a value of 1, i.e. the open loop control was effected in the last loop. If the feedback control was effected in the last loop, it is judged at the step S203 that the feedback control should be continued, and the flag F is set to a value of 0, followed by termination of the program.

If it is determined at the step 202 that the open loop control was effected, the program proceeds to a step S204 in which it is determined whether or not the transmission is in the first speed position. If the transmission is not in the first speed position, a first subtraction value ΔP_(BST) is obtained at a step S205 from a ΔP_(BST) table applied in a position other than the first speed position, in accordance with the change rate ΔP_(B), followed by the program proceeding to a step S207. FIG. 30 shows an example of the ΔP_(BST) table, in which two predetermined values ΔP_(B1) and ΔP_(B2) (ΔP_(B1) <ΔP_(B2)) are provided as the change rate ΔP_(BST). The predetermined values ΔP_(BST3) -P_(BST1) are set such that as ΔP_(B) is larger, i.e., as the increase rate of the supercharging pressure is higher, the first subtraction value ΔP_(BST) is set to a larger value.

If it is determined at the step S204 that the transmission is in the first speed position, the first subtraction value ΔP_(BST) is set to a predetermined value ΔP_(BSTF) applied in the first speed position. The predetermined value ΔP_(BSTF) is set at a larger value than the value ΔP_(BST) obtained from the ΔP_(BST) map applied in a position other than the first speed position.

Then, it is determined at the step S207 whether or not the intake pressure P_(B) is higher than the difference P_(BREF) -ΔP_(BST) between a desired value P_(BREF) and the first subtraction value ΔP_(BST) obtained at the step S205 or S206. The difference P_(BREF) -ΔP_(BST) is hereinafter referred to as "duty ratio control-starting pressure". The desired value P_(BREF) is set in accordance with the engine rotational speed N_(E), the intake air temperature T_(A), and the gear position of the transmission by the program of FIG. 24, as hereinafter described.

If it is determined at the step S207 that the intake pressure P_(B) is below the duty ratio control-starting pressure P_(BREF), a proportional control term D_(R) and an integral control term D_(I), which are applied to the feedback control, are both set to a value of 0.0, at steps S208, S209, and the duty ratio D_(OUT) is set to 100% to make the space area between the movable and stationary vanes 54, 49 the minimum, at a step S210. Thus, when P_(B) ≧(P_(BREF) -ΔP_(BST)), the space area between the movable and stationary vanes is set to the minimum, as at the period between t0-tA in FIG. 38. In this way, the increase rate of supercharging pressure in a low range is made the maximum so as for the supercharging pressure to be quickly increased to the desired value, thereby enhancing the responsiveness of the supercharging control.

Next, at a step S211, a t_(FBDLY) timer for delaying the feedback control is reset, and then the program proceeds to a step S118 in FIG. 24 to supply the control valve 69 with a driving signal corresponding to the determined duty ratio D_(OUT), followed by termination of the program of FIG. 24.

Referring again to FIG. 29, if at the step S207 the intake pressure PB is higher than the duty ratio control-starting pressure (P_(BREF) -ΔP_(BST)), it is determined whether or not the transmission is in the first speed position, at a step S212. If the transmission is in a position other than the first speed position, a second subtraction value ΔP_(BFB) is determined from a ΔP_(BFB) table applied in a position other than the first speed position, in accordance with the change rate ΔP_(B), and then the program proceeds to a step S215, hereinafter described.

FIG. 31 shows an example of the ΔP_(BFB) table, in which, just like the table of FIG. 30, predetermined values ΔP_(BFB3) -ΔP_(BFB1) are provided ΔP_(BFB3) <ΔP_(BFB2) <ΔP_(BFB1)), which are set such that as the change rate ΔP_(B) is larger, the second subtraction value ΔP_(BFB) is set to a larger value.

If it is determined at the step S212 that the transmission is in the first speed position, the second subtraction value ΔP_(BFB) is set to a predetermined value ΔP_(BFBF) for the first speed position, at a step S214, and then the program proceeds to a step 215. The predetermined value ΔP_(BFBF) is set at a value larger than ΔP_(BFBF) applied in a position other than the first speed position, determined at the step S213.

At the next step S215, it is determined whether or not the intake pressure P_(B) is higher than the difference (P_(BREF) -ΔP_(BFB)) between the desired value PBREF and the second subtraction value ΔP_(BFB) obtained at the step S213 or S214. The difference (P_(BREF) -ΔP_(BFB)) is hereinafter referred to as "feedback control-starting pressure". If the intake pressure PB is lower than the feedback control-starting pressure (P_(BREF) -ΔP_(BFB)), it is judged that the feedback control should not be effected, and then the program proceeds to a step S216 et seq. If the answer at the step S215 is no, that is, if (P_(BREF) -ΔP_(BST))<PB<≦(P_(BREF) -ΔP_(BFB)), open loop control is effected as at period between tA-tB in FIG. 38.

At the step S216, the t_(FBDLY) timer is reset, like the step S211, and at a step S217, it is determined whether or not the transmission is in the first speed position. If the answer is no, a subtraction term D_(T) is determined from a D_(T) table applied in a position other than the first speed position, at a step S218, followed by the program proceeding to a step S221, hereinafter referred to.

FIG. 32 shows an example of the D_(T) table, in which predetermined values D_(T1) -D_(T3) (D_(T1) <D_(T2) <D_(T3)) are set such that as the change rate ΔP_(B) is larger, the subtraction value D_(T) is set to a larger value, just like the map of FIG. 30.

If at the step S217 it is determined that the transmission is in the first speed position, a subtraction term D_(FT) is determined from a D_(FT) table for the first speed position in accordance with the change rate ΔP_(B), at a step S219. FIG. 33 shows an example of the D_(FT) table, in which two predetermined values ΔP_(BF1) and ΔP_(BF2) (ΔP_(BF2) <ΔP_(BF1)) are provided as the change rate ΔP_(B), and predetermined subtraction values D_(FT1) -D_(FT3) (D_(FT1) <D_(FT2) <D_(FT3)) are set such that as the change rate ΔP_(B) is larger, the subtraction term D_(FT) is set to a larger value. These predetermined values D_(FT1) -D_(FT3) are set at larger values than respective corresponding values D_(T1) -D_(T3) of FIG. 32 at the same change rate ΔP_(B).

As described later, the duty ratio D_(OUT) during the open loop control is set to a smaller value as the subtraction terms D_(T), D_(FT) are set to larger values. Therefore, by setting the value of D_(FT) to a value larger than the value of D_(T) in response to the change rate ΔP_(B), the rising speed of the supercharging pressure can be suppressed in accordance with an actual change in the supercharging pressure when the automatic transmission is in the first speed position. Therefore, with the aid of the basic dutV ratio D_(M) set, depending on engine operating conditions, an abrupt increase and overboosting in the supercharging pressure can be positively prevented when the transmission is in the first speed position, as indicated by the solid line I shown in FIG. 39, while the rising rate in the supercharging pressure can be controlled to a larger value when the transmission is in a position other than the first speed position to thereby obtain desired accelerability, as indicated by the chain line II in FIG. 39.

Then, the subtraction term D_(T) is set to the determined value D_(FT) at a step S220, and the flag F is set to 1 to indicate that the open loop control should be executed, at a step S221, followed by termination of the program.

If at the step S215 it is determined that the intake pressure P_(B) is higher than the feedback control-starting pressure (P_(BREF) -ΔP_(BFB)), it is determined at a step S222 whether or not a predetermined period of time t_(FBDLY) has elapsed after the t_(FBDLY) timer was reset at the step S211 or S216. If the predetermined time period t_(FBDL) has not elapsed yet, the program proceeds to the step S217 wherein the open loop control is executed, while if the time period t_(FBDLY) has elapsed, it is judged &hat the feedback control should be executed, and then the program proceeds to a step S223. In this way, even when the intake pressure P_(B) exceeds the feedback control-starting pressure (P_(BREF) -ΔP_(BFB)), the feedback control is not executed immediately, but the open loop control is executed until the predetermined time period t_(FBDLY) elapses, as at period between tB-tC in FIG. 38. Only after the lapse of t_(FBDLY), the feedback control is started, as at tC in FIG. 38.

At the step S223, an initial value of the integral control term D_(T) is calculated by the following equation:

    D.sub.I =K.sub.TATC ×D.sub.M (K.sub.MODij -1)

where K_(MODij) is a learned correction coefficient (learned value) calculated during feedback control in accordance with the program of FIG. 24, as hereinafter described.

Then, the program proceeds to the step S203 to set the flag F to 0 to indicate that the feedback control should be executed, followed by termination of the program.

Referring again to FIG. 24, at a step S107 following the step S106, it is determined whether or not the flag F has been set to 1 in the subroutine of FIG. 29. If the flag F has been set to I, that is, if the feedback control should be started, the desired value P_(BREF) is determined from a P_(BREF) map in accordance with the engine rotational speed N_(E) and the intake air temperature T_(A), at a step S108. FIG. 34 shows an example of the P_(BREF) map, in which predetermined values N_(V1) -N_(V20) of the engine rotational speed N_(E) and predetermined values T_(AV1) -T_(AV8) of the intake air temperature T_(A) are provided and set in just the same manner as the K_(TATC) map mentioned before. By the use of the P_(BREF) map, according to which the desired value P_(BREF) is set to a higher value as the intake air temperature T_(A) is lower, the desired value P_(BREF) can be set to appropriate values to operating conditions of the engine.

Then, at a step S109, it is determined whether or not the transmission is in the first speed position. If the answer is Yes, a predetermined value P_(BREFF) is subtracted from the desired value P_(BREF) determined at the step S108, at a step S110 to set the desired value P_(BREF), followed by the program proceeding to a step S111. On the other hand, if the answer is no, the program jumps from the step S109 to the step S111. In this way, the desired value P_(BREF) is set to a lower value in the first speed position than in a position other than the first speed position.

By so setting the desired value P_(BREF), when the transmission is in the first speed position, the supercharging pressure is controlled to a smaller value than a value assumed in another gear position, during a steady state of the supercharging pressure, so that torque applied to the transmission gear is made smaller, as indicated by the solid line in FIG. 39, thereby enhancing the durability of the transmission, whereas in another gear position the supercharging pressure in steady state can be controlled to a desired higher value, as indicated by the broken line in FIG. 39.

At the step S111, the difference ΔP_(BD) (=P_(BREF) -P_(B)) between the desired value P_(BREF) and the actual intake pressure P_(B) is calculated, and then it is determined at a step S112 whether or not the absolute pressure |ΔP_(B) | of the determined difference ΔP_(BD) is larger than a predetermined value G_(PB) (e.g. 2 mmHg). The predetermined value G_(PB) is a value defining the insensitive pressure width.

If ΔP_(BD) ≧G_(PB), respective constants K_(P) and K_(I) of the proportional control term D_(P) and the integral control term D_(I), are read, respectively, from a K_(P) table and a K_(I) table, in accordance with the engine rotational speed N, at a step S113. FIG. 35 and FIG. 36 show these tables, respectively. In the K_(P) table, two predetermined values N_(FBP1) and N_(FBP2) (N_(FBP2) <N_(FBP1)) of the engine rotational speed N_(E) are provided, and predetermined values K_(P1) -K_(P3) (K_(P1) <K_(P2) <K_(P3)) of the constant K_(P) are provided, which correspond, respectively, to N_(E) <N_(FBP1), N_(FBP1) ≧N_(E) <N_(FBP2), and N_(E) ≧N_(FBP2). On the other hand, in the K_(I) table, two predetermined values N_(FBI1) and N_(FBI2) of the engine rotational speed N_(E) are provided, and predetermined values K_(I1) -K_(I3) (K_(I3) <K_(I1) <K_(I2)) are provided, which correspond, respectively, to N_(E) <N_(FBI1), N_(FBI1) ≦N_(E) <N_(FBI2), and N_(E) ≦N_(FBI2).

Then, the proportional control term D_(P) is set to the product K_(P) ×ΔP_(BD) of the constant K_(P) and the difference ΔP_(BD), at a step S114, and the integral control term D_(I) is set to the sum (=D_(I) +K_(I) ×ΔP_(BD)) of the integral control term D_(I) obtained in the last loop and the product K_(I) ×ΔP_(BD), at a step S115.

The proportional control term D_(P) and the integral control term D_(I) thus determined are substituted into the following equation to calculate the duty ratio D_(OUT) applied during the feedback control:

    D.sub.OUT =D.sub.M ×K.sub.TATC +D.sub.R +D.sub.I

Then, the calculated duty ratio D_(OUT) is subjected to limit checking to adjust same within a predetermined range, at a step S117. A driving signal corresponding to the duty ratio D_(OUT) is supplied to the electromagnetic control valve 69, at the step S118, followed by termination of the program.

When |ΔP_(BD) |<G_(PB) at the step S112 and hence the actual intake pressure P is substantially equal to the desired value P_(BREF), the proportional control term D is set to 0.0, and the integral control term D is set to a value of same obtained in the last loop, at respective steps S119 and S120.

Then, it is determined at a step S121 whether or not the transmission is in the first speed position. When the answer is Yes, a coefficient K_(R) is calculated by the following equation at a step S122:

    K.sub.R ×(K.sub.TATC ×D.sub.M +D.sub.I)/(K.sub.TATC ×D.sub.M)

where the coefficient K_(R) represents an amount of deviation of the supercharging pressure from the desired value due to variations caused during the mass production of the engine and the control system and/or due to aging change.

Then at a step S123 the coefficient K_(R) obtained as above is applied to calculation of the learned correction coefficient K_(MODij) by the use of the following equation:

    K.sub.MODij =C.sub.MOD /A×K.sub.R +(A-C.sub.MOD)/A×K.sub.MODij

where K_(MODij) of the second term on the right side is a value of K_(MODij) obtained in the last loop and is read from a K_(MODij) map, hereinafter described, in accordance with the engine rotational speed N_(E) and the intake air temperature T_(A). A is a constant, and C_(MOD) is a variable which is set to a suitable value experimentally selected from 1- A.

The ratio of K_(R) to K_(MODij) varies depending upon the value of the variable C_(MOD). Therefore, by setting the value of C_(MOD) to a value falling within the range of 1- A according to characteristics of the supercharging pressure control system, the engine, etc., the value of K_(MODij) can be calculated to an optimal value.

Then, the learned correction coefficient K_(MODij) calculated as above is stored into the K_(MODij) map which is provided within a back-up RAM of the control unit C, at a step S124, and the program proceeds to a step S116 et seq. and is then ended. FIG. 37 shows an example of the K_(MODij) map, in which, like the K_(TATC) map of FIG. 28 and the P_(BREF) map of FIG. 34, the K_(MODij) value is classified into a plurality of predetermined values in accordance with the engine 75 rotational speed N_(E) and the intake air temperature T_(A). The value of K_(MODij) is calculated and the calculated value is stored in each of a plurality of regions defined by N_(E) and T_(A).

When it is determined that the flag F is equal to 1, that is, when the open loop control should be executed according to the subroutine of FIG. 29, a value of the learned correction coefficient K_(MODij) is read from the K_(MODij) map in accordance with the engine rotational speed N_(E) and the intake air temperature T_(A), at a step S125, and the proportional control term D_(P) and the integral control term D_(I) are both set to 0.0, at steps S126 and S129.

Then, the duty ratio D_(OUT) applied during the open loop control is calculated by the following equation:

    D.sub.OUT =K.sub.TATC ×K.sub.MODij ×(D.sub.M -D.sub.T)

where D_(T) is the subtraction term set at the step S218 or S220 of the subroutine of FIG. 29.

Then, the duty ratio D_(OUT) calculated as above is subjected to limit checking to be adjusted within a range from 0% to 100% at a step S129. This is followed by execution of the step S118 and termination of the program.

Although the embodiments described above are applied to a variable capacity turbocharger which has its capacity varied by means of movable vanes 54 as increase rate-varying means, the method of the invention may also be applied to other types of variable capacity type turbochargers such as a waste-gate type and a supercharging pressure-relief type, as well as to other types of superchargers than the turbocharger. 

What is claimed is:
 1. A method of controlling supercharging pressure in an internal combustion engine having a transmission and a supercharger, wherein the supercharging pressure created by said supercharger is controlled in dependence on operating conditions of said engine,the method comprising the steps of:(1) detecting a rate of rise of the supercharging pressure in a transient state; (2) detecting a gear position of said transmission; and (3) when the detected gear position is a predetermined lower speed position which is a first speed position, correcting the rate of rise of said supercharging pressure in said transient state to a value lower than a value assumed when the detected gear position is a higher speed position.
 2. A method as claimed in claim 1, wherein the supercharging pressure created by said supercharger is controlled based on a basic control amount determined in dependence on operating conditions of said engine, said basic control amount being corrected in response to a difference between an actual value of the supercharging pressure and a desired value of same, when said engine is in an operating condition in which the supercharging pressure is controlled in feedback control mode, and said step (3) includes setting said desired value of the supercharging pressure to a value lower than a value assumed when the detected gear position is a higher speed position, when said engine is in said operating condition in which the supercharging pressure is controlled in feedback control mode.
 3. A method as claimed in claim 2, wherein said step (3) is executed when said engine is in a predetermined operating condition.
 4. A method as claimed in claim 3, wherein said predetermined operating condition of said engine is a state in which intake pressure in said engine is higher than a predetermined value.
 5. A method as claimed in claim 4, wherein said predetermined intake pressure is determined in dependence on the rotational speed of said engine. 